A person's enthusiasm for life can make quite a difference to how they feel...at that time. If you're looking forward to something pleasurable you seem to be imbued with more energy, more zest for life. On the other hand, if you're bored, that certain ennui seems to bring you down and possibly makes you feel older than you really are. That's part of it.
Another part is that we have daily, (diurnal) and monthly bio-rythms, the latter being experienced particularly by those of the 'fairer' sex. These bio-rythyms, I'm told fall into three catergories: physical, emotional, and -quite frankly - I forget the last. When our physical bio rythym is operating at its peak we're able to 'pump iron' at the gym at our maximum capacity, when it's low, we seem to be actually physically weaker.
Are we only as old as we feel? I guess the answer has to be a sort of qualified, "yes." And this seems to stand no matter what our age. I'm seventy-four and was at the gym only yesterday. But then again, I've been training on and off since I was twenty.
My suggestion: get enthused with a project bigger than yourself. That'll keep you young.