I'm only 24, but I'd tell a younger me to just live life as fully and completely as possible. Work hard. Party Hard. Eat right. Love and protect those dear to you.
I missed out on so much when I was younger simply because I was too afraid to take chances. Or rather, too afraid of screwing up/being rejected etc. I realise now that making mistakes is just another part of life.
If I knew then what I know now, I probably wouldn't have still been a virgin at 21.
That said, I'm glad I waited. Weird huh.