Simply put, aging is a bitch.
I can't pull all-nighters or concentrate as hard as I could in my youth anymore, among many other things, and I'm more or less constantly exhausted simply due to the fierce but uncaring passage of time.
>mental rather than physical
No doubt it's more mental than physical, but it all comes down to aging one way or another. There's less wonder in life the more I experience; I'm just tired and the only explanation is I'm old, full stop. Most people are more young at 35 than I am, but I'm not one of them.
When I say I'm tired, I mean literally so. I have lots of joys in life even if it's not as wonderous anymore; lots of things I want to do. The problem is I don't have nearly enough energy nor time for them anymore as I get older. I have more responsibilities (things that need doing) as I get older and my body simply isn't as lively as when I was younger.
And that is only going to get even worse as time passes.
Aging is a bitch.
(For context, I'm 40 and feel old too. My partner died after a short and nasty illness, which was difficult in itself, and I was left to care for our then two years old kid. I only have one responsibility, but it's more than enough...)