A friend of mine once said (I’m not sure if he was quoting someone) that “maturity isn’t about being serious all the time, it’s knowing when to be serious and when to have fun.” I think that is somewhat in line with your motto.
I think if you don’t “behave youthfully” (go out, do stuff, take care of yourself, stay fit, stay reasonably current with trends and culture. hang out with friends) at some point you will feel so out of touch with these things that you will feel old long before you are actually old.
OTOH- I recall the OP starting this thread where he has no career to speak of, no savings, no assets, no real understanding of financial matters, no kids, and is finally in a stable relationship at 40. I would suggest that one reason they might feel “old” is that most 40 year olds would tend to be a lot further along in life at that point. It’s one thing to not have your shit together in your 20s. Usually by their 30s, people start to feel the passage of time in that you don’t have all the time in the world to figure stuff out.
IOW, you don’t want to work yourself into old age. But you also don’t want to be the guy hanging out at the high school / college parties years after graduation (so to speak).