I have come across several advice posts targeted towards younger individuals, and sometimes I find myself thinking, "I wish I had known these things earlier in my life."
However, there are certain pieces of advice that are applicable to people of all ages, such as maintaining clean eating habits, engaging in regular exercise, fostering healthy relationships, and building a strong network of friends and acquaintances.
Considering this, what kind of advice can individuals in their 60s offer to those in their 40s?
Work: find your niche. Accept that you're not going to be a hotshot coder/consultant/whatever forever. However, outside of 'fashionable tech' there is an entire world where you can make a comfortable living and nobody cares about your age. That doesn't mean you can stop learning: remaining up-to-date on relevant skills is important, but 'relevant' does a lot of heavy lifting there. Be knowledgeable/reliable instead of trendy.
Leisure: don't put off things you truly want to do until some unspecified later date. You might not make it (see 'health' below). Find at least one activity to enjoy on at least a weekly basis, travel (even if it's close to home), go for walks, spend time with your loved ones.
Health: everything truly and rapidly gets worse once you're 50. Stop smoking now, stop drinking now (or at least moderate a lot), talk to a doctor on a regular basis, take care of your teeth (really).
(That last paragraph is awfully generic, but until such time that we invent time machines, a very important one...)