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IIRC The Space Wolves have the oldest recruits, but even these are mostly pre-teen.

Pre-Heresy it was possible to 'enhance' adults (such as Kor Phaeron) but the knowledge has been lost by 40k.

pm713 wrote:
From what I remember a Marine is immortal in the sense that they don't die of age and if you left them on a farm and nothing bad happened they would be there in thousands of years.

Yes - Oberdeii, for example (and wasn't there a Heresy Era Salamander who was left chained up for thousands of years?).

He might have been in a sus-an coma when they found him - I can't remember.

There's also possibly Rylanor, but he's a Dreadnought and we don't know exactly when he died.