Mine date back to when I was about 12 years old. The family Doctor said they would probably go away on their own in time. I am 81 and still waiting. On the positive side, I grew into a guy with a very large chest so my breasts do not appear overly large on my 44 inch chest. even so, I buy jackets in the 46 to 48 range.
Last year I had a triple bypas and checked off everything in the list of things that might go wrong. Well actually all but one, I didn't die. But I did have multiple organ failures and between them and all the various medications I was taking at the time I went through a period of time when my breasts were highly sensitive in an erotic sort of way. I thought they had grown a bit in the process but my wife says no, they are still the same. Lets just say that to me they feel heavier.
But I went through hell for over a month during which time I was on a large number of drugs. anyhow, I survived it and gradually my labs came back to pre-operative levels.
The point is that for most of us, Breast growth is an event and we do not realize it until after the fact. There is a long list of possible causes, but the most common is simply "I don't know."