Most space marines are selected when they are roughly 10-13 years old and go through close to ten years of training and surgeries before they are even made into scout marines. Even the traitor legions, if they are capable of recruiting in the traditional manner and have the resources to perform the surgeries and arm the new brother would probably select recruits at the same age. Or some legions are not above accepting corrupted loyalists into their ranks. The deathguard I believe allows outsiders as long as they swear to Nurgle and Mortarion. 

That would still probably put you in the low hundreds in terms of age. like 120-300 years at least