When/at what age did you first notice stretch marks?

I'm 23F and I've always had a love-hate relationship with my stretchmarks. I've never had a significant amount of weight gain or loss, always an average size. But I first developed stretchmarks when I was about 14-15. I had never heard of them, I literally ran down to my mother thinking I was dying. I do have a condition that affects my collagen (I didn't know this at the time) so idk if that's why I had them so early? They've faded a lot so I don't mind them so much, but sometimes I worry just a little, especially during the summer time.

But what was it like for you? How old were you? Did you know what they were?