The youngest I have ever heard of was 11 years. I am 12 and understand how hard it is. Please get professional help to take care of the baby (if you have one). Give your son/daughter a good life and dont tell the father, especially if he is a grown man and you are young.
There was something online about a 5 year old in Peru but I doubt it's credibility. Heres a link:
http://www.incrediblebirths.com/Worlds-Youngest-Mother.html