Common experience suggests that ideological development is usually completed by the early twenties; after that age most people are ‘set’; regression is common but further development rare. Here is some evidence confirming that impression and suggesting that development is sometimes completed even sooner than one would have thought.
Lenin was 47 when he made his revolution in 1917; he was 23 when he first joined an overtly revolutionary organisation; he was 17 when he first identified himself as a person committed to a revolutionary purpose. Trotsky and Stalin were both much younger men; in 1917 they were both 38; but Trotsky had first joined a revolutionary organisation at the age of 23 and Stalin at 22; Stalin had first committed himself to the idea of being a revolutionary at 18, Trotsky at 17. For Bukharin, Zinoviev and Kamenev the picture is the same; respectively they were 31, 47 and 35 in 1927; they became permanent members of revolutionary movements at 24, 20 and 22; and they committed themselves to revolution at 18, 18, and 17. In more general terms, the average age of the Bolshevik leadership in 1917 was 39; but the average age at which they had first committed themselves subjectively to revolutionary action, first defined themselves socially as revolutionaries, was 17. In a famous study of the leadership of the CPSU in 1930 Davis found that 82% of his 163 Communist leaders had been under the age of 25 when they first joined a revolutionary movement. The median age for joining was 20; and the median age of their first publicly noted radical activity was 18. (Philip Abrams, Historical Sociology 1983)
from Ideological Commentary 12, August 1984.