it depends a lot on how u feel... my dad is almost 50, but he looks young, he feels young, he's always looking for new exciting things to do... But I know ppl who are just 35 and they feel older!
Yeah I agree, I had a grandma who died relatively young, 63 I think but because of her health and stuff seemed old. Meanwhile I have a great grandmother on the other side of the family who is 98 but because she still healthy and sharp minded doesnt seem old.
So yeah I think 'feeling' has a lot to do with. Feel young, you'll 'be' young. I dont think 40 is old though, but probably verges on middle aged, although I'd probably set that at 50-55.
When I find it hard doing normal tasks, getting around, etc...then Ill be old. Coincidentally thatll also be the time suicide will be comming into play.