At the age of 15, my response may be overlooked. But concerning your question, I had noticed my Gynecomastia around the age of eleven. It wasn't noticeable much keep in mind I was born a large kid and kept that figure for years, but once I lost pounds it was more obvious. This was around the age of thirteen. But it depends really on the age that men or boys will develop Gynecomastia. It all has to do with really if they're going through puberty or not. If he starts to ask you why there's hair down there, or what's happening to my voice, it would be the right time to start doing your own investigation by looking for common symptoms of Gynecomastia.