I think there would be a limit in terms of how old a marine would be when they are sent to the DW as the marine gets older they are likely to get killed easier i believe as even space marines feel their age at some point.
If you plan on being old then its likely you would be on permanent secondment to the Watch.
Already, you exalt me for my triumphs, When I ask only that you remember me for my treacheries
Victory is nothing more than survival.
It carries no weight of honour or worth beyond what we ascribe to it.
If you wish to grow wise, learn why brothers betray brothers. - Khyron, First Grand Master of the Eighth Brotherhood.