"Dear Mrs. Whoeveryourare, I hope this letter finds you well. You are most likely married and probably have a few children. Am I right? If you have forgotten so soon what you knew at age 17, let me remind you. Be patient with your children, but discipline them when they need it. Give them opportunities you never had (if money allows) but don't spoil them. Above all, be there when they need you. Also, be a loving wife, but don't let your husband rule your every move. Don't give up your dreams of having your written works published and never forget me - your childhood self."
If I could somehow travel through time (where is a Tardis or a Delorean when you really need one?), I would give this letter to my 17 year old self.
"Dear childhood self, I know you won't believe me, but you're an awesome 17 year old and you'll just get better with age. You won't have 6 kids, 4 horses, 2 dogs, 2 cats and a Quakerkeet, but you will have one perfect baby boy. You will be patient with him...mostly. You will give him many opportunities you never had, and spoil him terribly, but that's okay, because he's a really great kid. You will find your soul-mate and best friend. I won't tell you his name, but when you meet a redhead from Sweden, stick to that man like lint in velcro. He's a keeper!
P.S. There really is life past 40. It's not a myth!"
On second thought, I probably wouldn't give myself this letter. We really hate spoilers, you know. Instead, I'd give her a hug and possibly a few winning lottery numbers. And a quick lesson on eyebrow maintenance. :)