One of my friends retired at 19. A retired boy genius.
His parents dragged him to numerous talk and radio shows, public speaking engagements (mostly them doing the talking) from when he was six to sixteen. He did two years of college, dropped out and used the remaining funds intended for his education to qualify for a loan to buy a warehouse and turn it loft apartments. He gave himself a small apartment, lets a firm handle the rentals and has lived there ever since... enough money coming in that he doesn't have to want for anything but not really doing anything at all.
a former mate became famous for a startup. Sold it eight years later. He's also quite low key but had a more visible life. . . published a few books, did the socialpreneur thing for a bit and is something of a cult leader (all that sacchraine, poppy, positive affirmation shit) and techevangelist on twitter.
Yet another was part of a successful ensemble tv show, however, he was odd looking (not bad looking but unconventional for hollywood), tall (towers over most leads) and publicly out. So, being over twenty years ago that left pretty much left him to bad camp to softcore films, occasional theatre part and the odd guest billing - mostly on tv crime shows; enough to keep your name floating out there but not really enough to pay the bills. He went back to school and studied up to become an intimacy coach and therapist. On set, he's responsible for choreographing sex scenes to make them appear realistic while helping to actors to feel safe and boundaries respected to the regulations of the laws of the states he's allowed to work in (not all coaches are limited to that but the licensed psychs and social workers are more limited on this factor but get better pay plus industry clients).
Musicians, singers, tech... they fade into the background, usually still working but without the support of a major label and stuck on the festival scene or touring out of pocket. Quite a few end up as performers for benefits...fundraising. tho, often in more socially conservative circles; i.e. museums and the like that get upset when they've hired them because of the performers they are but triggered by their music. You can make a fairly decent living especially if you have tech skills or capable of working as session musician but it becomes a bit of a grind. And, there's far more bullshit in it than being on the road. But inevitably, most people settle in one place. songwriting for other bands/performers happens usually more in the immediate unless you have an established history for it but the more stable is in advertising or cataloguing sounds (or doing commissions) for content creators to use for a nominal fee or to buy licensing straight out. But it's highly competitive anymore and simulation software ( a lot of it is open source) is starting to eliminating it as it's massively improved in the last few years from the robotic ones you might be used to.
In short, you get a day job, if you didn't have one to begin with., If you're recognizable you do conventions, reality shows, gossip or tell-alls, and just generally whore out your dignity to keep your head above your ass. Many will disappear into family life with kids unaware they were ever celebs. Quite a few of those end up being a stay at home parent that may work remotely or from home. Maybe more the roadies than anyone. Stage performers - coaching of some kind is the popular avenue, probably setting up a social media account to dispense advice and attract wannabes willing to pay for a one on one. All will inevitably become old grumps shaking their fists at the sky over the next wave of performers... and probably wasting their spare time here because friends disappear quickly when the celeb does or your inability to switch off alienates everyone.