Ha I'm already 30+ - the fact, OP, that you've lumped me into the "older person" category...stings like a bitch!
Two of the wisest pieces of advice that my mother always regurgitates, are well known cliches.
1) Never be a borrower or a lender.
2) God helps those who help themselves.
Both are very very sensible pieces of advice.
The most recent piece of advice from someone older than myself which I received, isn't quite as profound, but still very helpful. I was a special guest at a convention last month and my table was next to Virginia Hey's table. I'd not met her prior to that occasion, and it was my first convention. She told me she always carries hand sanitiser with her to conventions, as you're always having to shake hands with people, handle money, and so on. She said she always picks up either a bad cold or a stomach bug if she doesn't take her hand sanitiser. I thought it was a very good idea.