For casters at least, at level 17-18, you're God. You can take any form you want, create a world from nothing, create a permanent living breathing being from dirt with two actions, create a second permanent living breathing being from a piece of the first living being, and quite a few other things. At this point, it might be more rewarding to retire to a world of your own creation.
For casters at level 21 actually using epic spellcasting rules ... you're the DM. After reaching a bit past this, as their last pursuit before retirement, I literally did hand the reins to the epic spellcaster in the party (there were two but one didn't bother with epic spells), who built an arena for epic warriors out of those spells where they could fight each other without risking these extremely valuable lives.
8-13 are our preferred levels pretty much, though.