Oh sure, once each primarch was found, their loyal followers were 'uplifted' to Space Marine status, which indicates that at least the Emperor and the Grand Crusade Era Imperium at least had the know-how to modify adults. We can extrapolate from there that chapters could very well have preserved that lore, either exclusively (thus losing the "preferred" method of augmenting children) or in addition too whatever the default is.
OF course, to a space marine of several centuries of age, even a man of 50 is 'young'...