When did your breasts start to sag?

I have no one to ask this to since my mother has passed and I don’t have any female relatives I am close to, so maybe some random ladies on Reddit can be my motherly stand-in.

I’m 24 and in the last year I have noticed a significant change in my breasts: they are no longer firm and perky to the point there is a clear fold where the breast/torso connection is. I knew this was going to happen eventually but I thought I had another 5-10 years.

Although I had two breast exams which came back normal, I’m curious to know if this is an age appropriate time for this to happen or if I should look at other reasons for it.

I’m somewhat active, I wear non push-ups and sports bras about 60% of the day, I don’t drink or smoke, never been pregnant, idk what else would be useful information…

ETA: bra size 36C