I never saw mood as related to age.
I see there are old teenagers... grumpy teenagers... others young at heart, always cheerful.
I see there are old adults... grumpy adults... others young at heart, always willing to help... to live, love, laugh.
Mega-veteran journalist Andy Rooney retired at 92. Both grumpy and cheerful.
I have a cheerful 8 year old, who is sometimes melancholic. And I have a bossy daughter, she's nine and she is my mother.
I'll be eternally 28 ;-) and always happy, although sometimes sort of absent. I just don't think age is a factor when it comes to personality.