I've had it since around 15 as well, didn't touch weed until early 20's so it definitely didn't cause it. About 5 years ago I gained a bit of weight which I think added to the problem but even after slimming down again, that still remained.
From what I have read, smoking MJ every day can affect testosterone levels and any extra is converted to estrogen, the hormone that can trigger breast tissue to grow, especially if someone has a few pounds on the belly. Smoking a spliff on occasion is unlikely to cause regrowth.
I think the more important thing is to live healthy, ie eat well, exercise and keep stress under control (stress releases cortisol causing excess body fat which can lead to more estrogen).
So, in my humble opinion, if someone puts the effort in to eat healthy, stay in shape, find ways to manage stress (yoga for example) than smoking a joint from time to time will not trigger regrowth.