And this other study comes to the opposite conclusion:
"People are least happy in their 40s and early 50s. They reach a nadir at a global average of 46".
I don't necessarily know what it would be, but I'd take the results of the poll with a grain of salt, especially considering this contradictory link.
I asked my wife's grandfather on his 88th birthday: Forget the mirror, how old do you feel on the inside? His answer: 33.
As a 48 yo, I gotta tell you I wouldn't believe I'm mentally past 22 without the mirror. But maybe that just means I'm incredibly immature. :)