To me, age 12.
"Life is going to get strange for you, kid. You're going to fall in love, and you're going to love her for a year. She's going to change you big time. But promise me this: Don't let women take advantage of you. You'll find out what I mean when you're 14. You can't say no to women, it just isn't in your nature. Also, the girl you will still want months after she left you, she doesn't want you anymore, as far as I can tell. But you're going to keep wanting her, because you're more than likely in love with her, and don't realize it. I'll try not to mess it up, but if I do, make sure you don't."
Hell, thats a bit pathetic, but it's the truth.
Or I'd do this.
"If you are receiving this message, a bucket of water is going to soak you." But there will be no water bucket!