Men in their 40s, what’s one piece of advice for men in their 20s?

I'm 39, but this is mine: do you just feel kinda "blah" all the time, don't enjoy anything including things you used to enjoy, and can't motivate yourself to do anything? That might be depression, and it might also be undiagnosed ADHD. The sooner you learn about that and get help with it, the better you'll feel and the more effective you'll be (and the less you'll let down the people you love).