By the time you graduate college, you only have 50ish years of good health left if you take care of yourself. That may sound like a lot to some of you younger folks, but during those years, a third of your day will be spent sleeping, another third of your day will be spent working, and most of the remaining third will be spent on commuting, chores, grocery shopping, home maintenance, and the like.
That leaves you with precious little time to do the things you really enjoy, and there are literally thousands of fun hobbies and activities out there. You can casually dabble in a tiny fraction of them, and before you know it, all your free time in the week is gone, and you have house work and paid work that needs to get done. Blink an eye, and the year is gone. Blink a few more times, and you're an old retired person with more time but no energy to climb that mountain, surf that wave, or backpack around that quaint faraway region of the world that you've always wanted to immerse yourself in but never had the vacation time for.
Blink once more, and you're now on your deathbed wishing you ran that ultramarathon or took that motorcycle or RV trip across the country. Or maybe you're a homebody who aspired to compose a one-hit wonder or write the next Great American Novel. Either way, your life is over, and you'll never have the chance again.
With that said, why the hell would you want to spend your very limited time on Earth following the conventional lifescript of marriage, McMansions, a 5-decade career, and 2-3 kids? F*** that, I'm retiring before 50 and will engage in as much of my hobbies as I can, while I still can.