My age!
Dunno, actually. I think it depends on the person and what is considered old. Example- my mother. We joke about her birthday being a leveling up every year, like she's hit "level 75".
Mostly because -while she has the gray hair and some wrinkles, she just has insane energy and spends every day all day doing stuff.
She'll text (family group txt) and say she weeded a garden spot, planted some flowers, picked some figs, walked her doggo, clean in the house, and it's like 9am! And she still has a full day planned. She helps drive "old people" to their appointments sometimes. They're usually in their 90s.
But then I've seen way less active people who just seem way older, and they're like, late 40s! So I think it depends on the person and their perspective.
I do remember thinking when i was in grade school that 30 was "so old".