Answer by taking a quote from the following article:

Lionel Barrymore was born Lionel Herbert Blythe in Philadelphia, the son of actors Georgiana Drew Barrymore and Maurice Barrymore (born Herbert Arthur Chamberlayne Blyth). He was the elder brother of Ethel and John Barrymore, the uncle of John Drew Barrymore and Diana Barrymore and the great-uncle of Drew Barrymore, among other members of the Barrymore family. He attended private schools as a child, including the Art Students League of New York. While raised a Roman Catholic, Barrymore attended the Episcopal Academy in Philadelphia.

Reluctant to follow his parents' career, Barrymore appeared together with his formidable grandmother Louisa Lane Drew on tour and in a stage production of The Rivals at the age of 15. He later recounted that "I didn't want to act. I wanted to paint or draw. The theater was not in my blood, I was related to the theater by marriage only; it was merely a kind of in-law of mine I had to live with." Nevertheless, he soon found success on stage in character roles and continued to act, although he still wanted to become a painter and also to compose music. He appeared on Broadway in his early twenties with his uncle John Drew Jr. in such plays as The Second in Command (1901) and The Mummy and the Hummingbird (1902), the latter of which won him critical acclaim. Both were produced by Charles Frohman, who produced other plays for Barrymore and his siblings, John and Ethel. The Other Girl in 1903-04 was a long-running success for Barrymore. In 1905, he appeared with John and Ethel in a pantomime, starring as the title character in Pantaloon and playing another character in the other half of the bill, Alice Sit-by-the-Fire. In 1906, after a series of disappointing appearances in plays, Barrymore and his first wife, the actress Doris Rankin, left their stage careers and travelled to Paris, where he trained as an artist. He did not achieve success as a painter, and in 1909 he returned to the US. In December of that year, he returned to the stage in The Fires of Fate, in Chicago, but left the production later that month after suffering an attack of nerves about the forthcoming New York opening. The producers gave appendicitis as the reason for his sudden departure. Nevertheless, he was soon back on Broadway in The Jail Bird in 1910 and continued his stage career with several more plays. He also joined his family troupe, from 1910, in their vaudeville act, where he was happy not to worry as much about memorizing lines. From 1912 to 1917, Barrymore was away from the stage again while he established his film career, but after the First World War, he had several successes on Broadway, where he established his reputation as a dramatic and character actor, often performing together with his wife. He proved his talent in such plays as Peter Ibbetson (1917) (with brother John), The Copperhead (1918) (with Doris), The Jest (1919) (again with John) and The Letter of the Law (1920). Lionel gave a short-lived performance as MacBeth in 1921 opposite veteran actress Julia Arthur as Lady MacBeth, but the production encountered strongly negative criticism. His last stage success was in Laugh, Clown, Laugh, in 1923, with his second wife, Irene Fenwick; they met while acting together in The Claw the previous year, and after they fell in love he divorced his first wife. He also received negative notices in three productions in a row in 1925. After these, he never again appeared on stage.

How old was he when he started acting on stage?
age of 15.