So the age of space marines varies throughout the Imperium. Some chapters are more long lived than otyers naturally but all space matines eventually grow old. Especially those with less stable geneseed.<br/><br/>The blood angels and their successors are actually unusual in that they tend to have an abnormal amount of veteran brothers who break that 1000 year mark, and old age seems to affect blood angels less than other lineages, which would be like you or me living to 100. But most space marines wont come near that figure. For a brother who isnt from sanguinius's line to live to 1000 is actually really rare. The odd chapter master or chief librarian perhaps, but most SM lineages seem to cap out around 8-900 as the extreme for their respective old age, again somenbreak that rule but generally that is considered really fucking old by space marine standards, and their reflexes will probably start to slip at that point.<br/><br/>But again, due to the nature of their job most space marines wont see that figure. They might live to see three centuries, maybe four before they get killed. Most candidates for deathwatch, particularly their first tour are probably into their second century or starting/mid way through their third.<br/><br/>By the time a brother finishes his training through the various reserve companies and gets a spot in a battle company. He is probably just coming up to 100 years. Like eighty something.