Thanks for clarifying. Being mindful of opportunity cost allows people to retire early, to spend more time with their loved ones, do better financial decisions.
I have seen too many of my loved ones work until their 70s because they didn't think about those concepts enough. That is what is sad.
Don't forget though there are things you won't be able to do at 50 even if you're retired. I'd hate to think of someone being a virgin hermit finally coming out the shell at 50 thinking the world is their oyster now. Some things you have to do when you are young.
My experience is that most decisions aren't of the type. It's not that you're choosing fun at 20 or longer fun at 50.
Investing or not. Renting a cheaper place for a while or buying a bigger house right now. A lot of the time you will get more money and more fun if you do it right.
Probably far fewer things than you think, though, as long as you're healthy. You can still have love affairs, travel the world, go to festivals, climb mountains, learn to dance well into your 70s or even 80s, as long as you are healthy. Sure, you might be climbing a small mountain instead of Everest, but small mountains are beautiful too.
Health in old age is something that you invest in, much the same as wealth. Of course you can suffer an accident, or lose your investments from bad luck. But in both cases you're guaranteed bad results if you don't put the effort consistently and starting when you're younger.
For health, this means making exercise, good sleep, and good diet a part of your daily routine. Much like investments, you can go as deep on that as you like, but if you start early all you need is basic knowledge.
Also regardless of the quality of the examples, it may be the case that the point (opportunity cost is important to consider) is still valid.