If you want your mind blown when Simpsons started Marge and Homer were boomers, then Gen X and now they're millennials. Bart, Lisa and Maggie started out as Gen X (underachiever and proud of it) then millennials, then gen z, and now they're Gen alpha.
The last time they mentioned his age it was 38/39. They keep changing it but keeping him and Marge in their 30s.
Honestly I wish they would age everyone up a year every decade. If they had done that we would be seeing the antics of a 13 year old Bart now and I think that could start with some really interesting character development instead of keeping him forever a 10 year old.