Early 30's (31, 32, 33) is still fairly young. People are still fertile (if that's a point of consideration), energetic, often haven't yet faced any age-reliant disabilities (arthritis, dementia, etc., although in rare cases these can be early onset), still have youthful looks and such.
It's around the age of 50 that the big changes start to take place, though some see these earlier on (around the 40's). This is around what I'd call middle age. Not young, but there is absolutely potential to still feel young and do those things you enjoyed in your youth.
Old I would say starts around the later half of 50's, or at about 60. Anybody above 60 I feel is unarguably old physically and possibly mentally, but not necessarily emotionally (see below).
Largely, age is a pretty social-reliant construct. You don't really have to be old at 60 if that's not how you feel. As the eras burn away and paradigms change, people in their 60's seem to be looking increasingly younger. I once mistook a 40 year old for about late 20's when I was 15. Even if that just means I'm terrible at guessing ages (I am, really), I still think aging is improving as time wears away.
On to the emotional part; some people (especially with autism, as I've seen on this forum) don't feel emotionally they represent their physical age. You can be 60 but still feel 16, become overly sensitive and upset at difficult events. Some of us mature but others do not.
So I suppose the question is maturity versus physical age. It depends on how you feel.