I would advise the 15 yr old to spend time with the older generation.
What regrets do they have, what would they do over?
What have they learned that made them wiser? What are they most proud of and why do they do things differently, think differently than they did at 15?
If I was one of the older and wiser than I would tell them to create their own regrets. Do things with the information they have at this very time.
Make mistakes and learn from them.
Fail and try again.
Learn all that they can, seek knowledge.
Live the life you were given and do the very best you can.
Only then can you become wiser.