Board Thread:Warhammer 40k General Discussion/@comment-18016240-20140707161751/@comment-6078851-20140707165816

the problem with chaos marine ages is that since they reside on warp-influenced worlds they may not be as old as 'real-time' suggests: time in the warp doesn't technically exist, or at least it doesn't exist in the same manner as realspace time, and this time distortion affects worlds in and around warp-ed regions - eye of terror, maelstrom etc. so while horus may have been around for 10,000 years he may have only lived for 500 years in 'realspace-time'.

bjorn was around at the heresy but he was almost killed, and since then he's been interred in a mobile life-support system :)

but in terms of real-time age and still fully functional then the award has to go to dante of the blood angels - 1,100 years and counting.