I would not say 40 is old.
Most people don't consider 40 years old to be old unless they are really young.
I used to think and consider the age of 40 to be old when I was 18 but now that I'm 33 years old I don't see 40 as old at all.
I'll be 40 years old in 7 years.
Also 7 years used to seem like a long time to me but now 7 years seems like maybe 2 years.
I also used to think the age of 30 years was old but now that I'm 33 I still feel pretty young.
I would say that once you get to 60 years or 70 years old then you're considered old.
My mom is 66 though and she says she still feels young and my dad is 73 this year and he still says he feels young.
Hopefully I will still feel young at those ages too.
Stay active, healthy and you may still feel young.