That would be freaky. Can you imagine if somebody woke up one day and they were a young adult? None of their friends and family would believe them, they'd think they were an imposter, unless they were able to do a comparison dna test from before and after.
I used to say I wanted to remain Twenty-seven, it's a good age, not too young, not too old, no pimples or awkward growing phases, and no wrinkles or grey hair yet, lol!