I'm in a weird spot here at 30 years old, but let's see...

My advice to ~~younger~~ people would be to take care of their mental health, and to do it via scientific practices.
For example, cognitive behavioral therapy has enough evidence of it working; therapy through spirits, don't. Medication can be necessary and its effectiveness is proven; that's not the case for extreme diets.

Also, philosophical counseling is a thing and it is good, but just like psychotherapy, it may not be enough. Sometimes we are dealing with mental disorders that require pharmacological treatment. Conversely, psychiatry and medication are there for people who need it, but sometimes we don't need it and we need better habits, better environment, counseling, etc. It is usually a combination of many things the way we can start feeling better.

I'll still read the advice from others because, well, I'm sure I can learn a lot from them.

Edit: I thought it was the other way around. Oh, well, it still applies. I wish my parents and other people their age would give mental health treatments a try.