Getting old is mandatory (for now at least, come one telomerase regeneration) but growing up is optional. You don't have to be one of the boring set that 20 years earlier was racing around on a motorbike but now is more interested in their new conservatory or kitchen remodel.
Equally depending upon what goes you might still be growing, your muscles might still be forming (training early might see peak achieved but most are in their 20s to about 30 when they fully develop), your bones are probably now starting to harden (sucks as you can't bounce as much, also
https://www.cappskids.org/skull-sutures-when-do-they-close/ ) and it is usually about 25 that people stop thinking they are immortal (see car crash stats and why you might have to be that old to join various elite police and military units) but also might vary in neuroplasticity (though this is debated at to how much and how it works, change happens either way).
EOF so song and as I already did I don't wanna grow up I will go with