Your friends are probably exaggerating, mate. Don't worry about what they say. Anything around five or six inches is considered average. Seriously, do a search on google and more times than not, you’ll see that 5-6 inches is normal. I could suggest other research, but I won't.
You haven't stopped growing yet. I can only assume a penis grows in proportion to the rest of the body, but I'll leave that for someone else to answer totally.