boobs sagging?

So it’s summer and i haven’t been wearing a bra 90% of the time. I have A cup boobs, and the area under my boob has never really been defined. Does that make sense? My boobs have always been low down, and there’s never really been a defined line under my boob where the boob ends. I’m a teenager, so young, but i’m scared my boobs are gonna start being saggy because i’m not wearing a bra. is that just a myth?