I will bite.
I didn't quite the personal finance habits early in life, but I am catching up now. The only good thing that I have maintained is that I never incurred consumer debt and student debt (college was paid for by parents, bless them).
At present, I am towards a path to financial independence by age 40 (all debts paid off, or at least have enough cash to wipe out all debts, net worth at least $0.5M and possibly more, estimated at 0.7M).
Some lessons to younger self:
Look to save money on rent. I unnecessarily blew away close to $40K on rent when I had perfectly good alternatives of sharing an apartment or even renting someone's basement.
Do not be afraid of the market: Seriously, my portfolio could easily be $50-100K more right now if I was not afraid of the market and really understood risk. I took foolish risks and in my fear, foolishly parked my money in money markets when the stock market was taking off.
Use money very strategically: Being social is important. Try to find the right set of friends, and spend money going out. However when alone, minimize. Make your own foods. Going out by yourself is a horrible waste of money and as a single person, you should not be excused for squandering money on that.
Do not be too social: It will drain emotional energy that's needed very much to focus on your own career. Seriously, 20s is the best time to really focus on your career, work very very smart (not just hard) and take off. In fact, serious relationships can be postponed until 30s. 30 is a crucial age. On your 30th birthday, you should know you have made it (I didn't, my career took off in my early 30s).
Become a smart judge of character: Getting a spouse who shares your major goals is of a paramount importance. Small details matter, but if you don't see eye to eye on money, you are headed to a cliff. Having the right respect for money is important. You have it, now get a partner who shares it with you.
Do not be afraid of the future, and for god's sake, don't act like you are 50 when you are 18 seeking help on reddit :) (This one's self explanatory).
More income will always trump more frugality.
Being frugal early on and paying your debts is strategic because it allows you later to enjoy more of your money.