Being 17 and because of my sexuality i have seen many penis's, between the ages of 10 upwards (note: i was the same age as them at the time). But it very much varys. But i would have to say that for a 15 year old the average is between 9cm (3-4 Inches) to 12cm (4-5 Inches). (PS im not sure with conversions there). But remember this is merely and average (of a 17 year old). And average, is not what it should be, it is simply what is common. If you do not fall into this gap, dont feel bad or wrong. For instance, i matured early, so i was larger than those sizes.
Hoping this is the info that you were looking for,