I hope it's okay to go a bit older than people in their 40s and 50s. I'm a family sociologist at Cornell, and we surveyed over 1200 people over 60 on this very topic (described in the book 30 Lessons for Living: Tried and True Advice from the Wisest Americans). Here are four "life lessons" they wanted to pass down to younger people - hope you find them helpful!
Recently, I led a roundtable panel with several fellow Gen X scholars/professors at an academic conference entitled "Generation X Turns 50." The discussions were funny, lively, engaging, candid, inspirational and very informative. In short, we kept it real! It was a pleasure to be part of such a vibrant discussion.