The most common concept of mind "creation" documented are GSVs physically constructing them and their first ship body. In some places it hints on a sort of ironic/humourous child/parent relationship.
I think Banks just wanted to write a particular type of story, and so he just air–brushed that possibility out so that people would stay people.
Also, don't lose sight of the fact that this utopia is really only possible because energy is literally free, in that any quantity of energy can simply be summoned into existence at any time, for no cost. And they never seem to have to plan for how to dissipate the waste heat, either. Again, he wrote a particular kind of story. Also, once we start building our Dyson swarm we'll be in a similar situation: https://www.youtube.com/watch?v=pP44EPBMb8A
I recall that it’s implied that a lot of members of the culture are hanging out in perpetual virtual reality, they just aren’t relevant to the story. As for a person becoming a Mind, I think the intention of the books is to convey that the gulf is uncrossable. A person is about as close to being a Mind as a grain of sand is, and new Minds are created exclusively by the existing Minds with care toward preserving the lineage of the Culture’s worldview and feeling of obligation toward stewardship for humans. It’s mentioned that Minds created without this sort of deliberate alignment-interference simply choose immediately to depart the reality that the Culture inhabits.
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.
Generally anything irreversible.
