It's not really about age, but about where you are in puberty.
The penis stays close to the same size from birth to puberty. Then, about 6 to 12 months after the start of puberty (when your balls got bigger and began producing testosterone), it starts to grow. Once it does start, it will get to its full adult length (determined before you were born) in about 18 months.
So, if you started puberty at 11, or early 12, your penis is probably fully grown. Most of us worry that we are too small, and almost every single guy on the planet would like an extra inch - regardless of how big he is. If you are measuring correclty, you are slightly over average for a developed penis. 65% of guys are in the 5 to 5 1/2 inch range. All but a very small percentage of the rest are within an inch of that average - so, 4 to 5 for about 16%, and the same number in the 5 1/2 to 6 1/2 range.
I had the same worries most guys do. Seeing about a half dozen of my friends hard during my teen years helped me feel more normal about my own development. If you compared with friends, you'd also probably find the same thing.