A key to happiness is having CHOICES. I think you're happiest when you know you have OPTIONS.
The Young are obviously happier because the future lies before them. Even if they're dirt poor, they still THINK they have choices. And they actually DO. The most important: Whom they're going to marry, where they're going to work, where they're going to live, etc. All lies before them. Even in the misery of a low-paying job, they still think/know that the current situation won't last forever. If they have a shitty lover -- that, too, can be changed. And they can move if they have nothing to keep them where they are.
The crappy part of growing older is that the choices start to shrink, partially because of your own self: You get tired of flitting about and understand that it might just be time to PICK someone and something. So you make your choices. And you are decidedly NOT free after that. With choices come intense consequences, the result of intermingling your own psyche with another's. You won't ever be the same again. For the lucky people, this is a GOOD thing. (I haven't been at all so lucky and so I'm cynical; but I've caught glimpses of what connecting feels like spiritually -- of course, I miss what I think that closeness simply MUST be like.)
Here's something from Sylvia Plath's "The Bell Jar," published in 1963, when she was 31 years old. Based on her experiences in New York City during an internship for "Mademoiselle" magazine when she was 20, only a few months before she first tried to kill herself:
“I saw my life branching out before me like the green fig tree in the story. From the tip of every branch, like a fat purple fig, a wonderful future beckoned and winked. One fig was a husband and a happy home and children, and another fig was a famous poet and another fig was a brilliant professor, and another fig was Ee Gee, the amazing editor, and another fig was Europe and Africa and South America, and another fig was Constantin and Socrates and Attila and a pack of other lovers with queer names and offbeat professions, and another fig was an Olympic lady crew champion, and beyond and above these figs were many more figs I couldn't quite make out. I saw myself sitting in the crotch of this fig tree, starving to death, just because I couldn't make up my mind which of the figs I would choose. I wanted each and every one of them, but choosing one meant losing all the rest, and, as I sat there, unable to decide, the figs began to wrinkle and go black, and, one by one, they plopped to the ground at my feet.”
I first read "The Bell Jar" when I was 15 or so. I didn't pick up on this passage at all. I didn't understand it until I re-read the book for perhaps the 6th time when I found it while doing laundry in the basement of my landlords' duplex in Weehawken, New Jersey, in 2008 after I'd moved to NYC a year earlier to make a new life, only later finding a haven in Jersey...
I understood the tragedy of "The Bell Jar" completely once I was in my 40s.