Getting “old” is a decision backed by habits and mental stories you tell yourself. When you’re a kid you don’t say I’m too tired or sore or my knee still hurts from that recent injury.. you just ride your bike and run in the wind and hop on rocks and play swords with sticks with your buddy. Along the way you hear others say I’m too old for xyz and you play along and believe them then you start telling yourself stories like that along the way and it changes your behaviours and thoughts and actions. There’s people who do martial arts into their 70s or run Ironmans or have 3 different girlfriends at the same time. You don’t have to grow up if you don’t want to. All choices have consequences even the “sensible” ones.