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Birth date from family Bible.

1900 U.S. Census for Baltimore City, Maryland:
Vol. 11, E.D. 76, Sheet 4, Line 86
Montford Avenue, Baltimore, house number 823
Charles Kopp, white, born June 1857, age 42, born in Maryland. Day laborer. Both parents born in Germany. He owned his house, which was mortgaged. Married 18 years
Minnie Kopp, wife, born June 1862 in Maryland, age 37. She had had 10 children, of whom 5 were living. Both parents born in Germany
William Kopp, son, born December 1881 in Maryland, age 18, boiler maker
Charles Kopp, son, born July 1886 in Maryland, age 13, at school
May Kopp, daughter, born October 1891 in Maryland, age 8, at school
Julia Kopp, daughter, born May 1895 in Maryland, age 5
John R. W. Kopp, son, born December 1897 in Maryland, age 2
All could read and write.

1910 U.S. Census, Baltimore City, Maryland:
Enumeration District 8, Ward 1, page 2B
2705 Eastern Avenue, Dwelling No. 30, Family No. 35
Donaldson, Robert C [sic], head, male, white, age 46, married twice, number of years of present marriage: 7, born in Maryland, both parents born in Maryland, speaks English, occupation traveling salesman, merchant tailor, wage earner, not out of work in 1909 or 1910, can read and write, owns house free of mortgage
Donaldson, Minnie, wife, female, white, age 48, married once [sic], number of years of present marriage: 7, mother of 10 children, 7 children living, born in Maryland, both parents born in Germany, speaks English, no occupation, can read and write
Kopp, Charles, step son, male, white, age 24, single, born in Maryland, both parents born in Maryland, speaks English, no occupation, can read and write, in school since 1 September 1909
Kopp, Julia, step daughter, female, white, age 15, single, born in Maryland, both parents born in Maryland, speaks English, no occupation, can read and write, in school since 1 September 1909
Kopp, John, step son, male, white, age 13, single, born in Maryland, both parents born in Maryland, speaks English, no occupation, can read and write, in school since 1 September 1909
Kopp, Frank, step son, male, white, age 10, single, born in Maryland, both parents born in Maryland, speaks English, no occupation, can read and write, in school since 1 September 1909
Donaldson, Cecilia, daughter, female, white, age 6, single, born in Maryland, both parents born in Maryland, no occupation

There are two different marriage dates from family Bible, both listed on the same page, so he must have married twice. The first is dated 6 April 1915. The second is 8 May 1919.

On 5 June 1917, John R. W. Kopp registered for the World War I draft, giving his full name as John R. W. Kopp; age 24; residing at Sexton St, Morrell Park, Md; born 6 December 1892; natural born; born in Baltimore City Md U.S.A.; tool maker for Bartlett & Hayward Machine[?] Co of Scott & Ramsey[?] Sts Balto City; dependents include wife and two children; married; white; no prior military service; claims exemption because of dependent; medium height, slender build; light brown eyes, brown hair; no disabilities. [World War I Draft Registration Card, Roll 1684199.]

On 10 May 1919, John R. W. Kopp married Katherine B. Holthauser in Baltimore City, Maryland. He was 23, resident of Baltimore, Maryland, white, no occuption listed, previously married, prior marriage dissolved “by death”; she was 19, resident of Baltimore, Maryland, white, no prior marriage. [State of Maryland, Department of Health, Report of Marriage (no index number).]

On 11 May 1919, The Baltimore Sun published Marriage Licenses, including:
KOPP — HOLSHAUSER — John R. W. Kopp, 23, divorced, 2705 Eastern avenue; Katharine B. Holshauser, 19.
[The Baltimore Sun, 11 May 1919, page C1.]

1920 U.S. Census, Baltimore City, Maryland
Enumeration District 108, 8th Ward, page 19A
2503 East Preston Street
Kopp, John R, head, renter, male, white, 22, married, not in school, can read and write, born in Maryland, both parents born in Maryland, speaks English, foreman in ship yard, wage worker
Kopp, Catherine, wife, female, white, 19, married, not in school, can read and write, born in Virginia, father born in North Carolina, mother born in Germany and speaks German, speaks English, no occupation

The 1921 Baltimore City Directory lists on page 1246:
Kopp, John R helper h 2503 e Preston

1930 U.S. Census, Baltimore City, Maryland
26th Ward, Block 237, Enumeration District 4-407, Supervisor’s District 4, Sheet 5A
Enumerated on 3 April 1930
3401-6 Baltimore Street
Dwelling 46, Household 52
Kopp, John R., head, renting for $38, male, white, age 32, married, first married at age 22, not in school, can read and write, born in Maryland, both parents born in Maryland, speaks English, occupation mechanic with electrical equipment, wage worker, at work previous day, not a veteran
Kopp, Katharine, wife, female, white, age 29, married, first married at age 19, not in school, can read and write, born in Virginia, father born in North Carolina, mother born in Germany, speaks English, no occupation
Kopp, James, son, male, white, age 9, single, in school, born in Maryland, father born in Maryland, mother born in Virginia, no occupation
Kopp, Betty, daughter, female, white, age 6, single, in school, born in Maryland, father born in Maryland, mother born in Virginia, no occupation
Kopp, Katherine, daughter, female, white, age 4/12, single, not in school, born in Maryland, father born in Maryland, mother born in Virginia, no occupation

1936 Baltimore City Directory, page 717:
Kopp John R (Kath H) shtmtlwkr h629 s Savage

1937 Baltimore City Directory, page 694:
Kopp John R (Kath H) sheet metal wkr h 629 Savage

John R. Kopp was mentioned in the 3 February 1938 will of his mother Minnie Donaldson. He was to receive one-seventh of her real and personal estate and was also to serve as her executor. The will was proved on 5 August 1939. [Baltimore City, Maryland, Wills, J.H.B. 193, pages 277–279.]

On 11 March 1939, John R. Kopp submitted an inventory of the personal estate of Minnie Donaldson. Total valuation of $3,666.68. The inventory was approved the same day. [Baltimore City, Maryland, Inventories, J.H.B. 277, page 238.]

On 1 July 1939, the administration of the estate of Minnie Donaldson by John R. Kopp, executor, was settled. The estate’s total valuation was $7,605.90, including $2,475.31 in cash and $3,600.00 from the sale of real estate. Two legacies were paid out at $25 each to William H. Donaldson and Robert L. Donaldson. The remaining portion was divided, one seventh to each of Sylvester Dekeen, Charles M. Kopp, Mary L. Young, Julia H. Davis, John R. Kopp, Frank C. Kopp, and Cecelia G. Korte (who owed rent to the estate). [Baltimore City, Maryland, Administration Accounts J.H.B. No. 358, pages 91–93.]

1940 U.S. Census, Parkville, Baltimore County, Maryland
Enumeration District 3-56A, page 12
7444 Park Drive, Household 39, owned, value of home $3,800, not on a farm
Kopp, John R. W., head, male, white, 42, married, not in school, highest level of education 4 years of high school, born in Maryland, resided in Baltimore, Maryland in 1935, did not live on a farm in 1935, employed for pay, worked 40 hours week prior to the census, occupation sheet metal in electrical implements industry, private work, worked 52 weeks in 1939, income $2,450, no other sources of income
Kopp, Katherine, wife, female, white, 39, married, not in school, highest level of education 2 years of high school, born in Virginia, resided in Baltimore, Maryland in 1935, did not live on a farm in 1935, home housework, no other income
Kopp, James F., son, male, white, 19, single, not in school, highest level of education 4 years of high school, born in Maryland, resided in Baltimore, Maryland in 1935, did not live on a farm in 1935, worked 40 hours week prior to the census, occupation aerospace mechanic for plane factory, private work, worked 52 weeks in 1939, income $1,000, no other sources of income
Kopp, Elizabeth K., daughter, female, white, 16, single, in school, highest level of education 1 year of high school, born in Maryland, resided in Baltimore, Maryland in 1935, did not live on a farm in 1935, not employed, no other sources of income
Kopp, Kay D., daughter, female, white, 10, single, in school, highest level of education 4th grade, born in Maryland, resided in Baltimore, Maryland in 1935, did not live on a farm in 1935

On 15 February 1942, John Robert Kopp registered for the World War II draft, giving his residence as 7414 Park Drive, Parkville, Baltimore County, Maryland; age 44; place of birth Baltimore, Maryland; telephone Ha.9277; date of birth 6 December 1897; name of persopn who will always know your address “Wife (Above address)”; employer Western Electric Company, 2500 Broening Highway, Baltimore, Maryland. Physical description: white; 5’8”; 150 pounds; brown eyes; both brown and gray eyes checked; light complexion. Signed John R Kopp. [World War II Draft Cards for Young Men, 1940–1947.]

On 18 April 1944 at 8:35 a.m., John R. Kopp died at Church House Hospital, located at Broadway and Fairmont, Baltimore City, Maryland. He had been hospitalized for 9 days, had lived in Baltimore 44 years, resided at 7414 Park Drive, Baltimore, Maryland. He was white, male, married to Katherine Kopp. Born 6 December 1899 in Baltimore, Maryland. Died aged 44 46 [sic] years 4 months 12 days. Occupation is illegible but may say plant mgt. Father was Michael Kopp, born in Baltimore, mother unknown, born in Baltimore. Informant was wife Katherine H. Kopp of 7414 Park Drive Avenue. Buried on 21 April 1944 at Parkwood Cemetery, Baltimore, Maryland. Attended by physician from 4 April 1944 to 18 April 1944. Cause of death partially illegible, subar--noid hemorrhage, duration 12 days. [Baltimore City, Maryland, Certificate of Death G16351.]

A death notice stated:
KOPP.—On April 18, 1944, JOHN, of 7414 Park drive, beloved husband of Catherine Kopp (nee Holshouser). Remains may be viewed at the funeral home of Leonard J. Ruck, 5305 Harford road, until noon Friday and the Asquith Presbyterian Church, Harford road and Oakcrest avenue, from 1 P. M. until the time of the service at 2 P. M. Interment in Parkwood Cemetery. [The Sun (Baltimore), 19 April 1944, page 17; 20 April 1944, page 21; 21 April 1944, page 17.]

A death notice stated:
KOPP.—A special communication of Mystic Circle Lodge, No. 109, A. F. & A. M., will be held Friday, April 21, 1944, at 1 P. M. at Masonic Temple, 225 North Charles street, to attend the funeral of our late brother JOHN R. KOPP.
LAWRENCE V. CORBETT
Worshipful Master
[The Sun (Baltimore), 20 April 1944, page 21; 21 April 1944, page 17.]

The Kopp family Bible states that John R. Kopp died in 1943.