It's mentioned in I think Excession, that some of the attitudes we see in the novels are just
current fashion trend. Yes, a long term fashion trend, but heavily a fashion trend.
Zakalwe is immortal by default, which is not something currently in, but he is nowhere close to real methuselahs of Culture.
One of the characters in Hydrogen Sonata has been present when the Culture was created, when the agreement that formed it was signed, and was part of the group that did the work. He still keeps the same external looks, but his body mass is nearly entirely computer storage to deal with his memories.
In Excession, one of the characters mentions how the fashion changed and how their family "did it all" - where all included things like living as sentient mist.
Another wants (and finally did it) to become biologically like the Affront. The opposition to it is not just that it's Unusual Life Choice, but more to the fact that he wants to join the Affront civilisation, and injecting someone permanently into another civilisation that is much less developed is not taken lightly - but happens (Similar deal is done in The State of the Art).
A lot of the people who die "of old age" in Culture seem to die through ennui - those who find interest to live on, easily do so. And social pressure depends on accepting it - IMHO later books quite easily show that it's only the matter of whether you care about it or not - and if the main character of Player of Games didn't care so much for the social standing they built for themselves, then SC would have had to find someone else - or devise a completely different method of getting them on board of the mission to Azad.