I agree with Remaker that while it's important to make sure a 14-year-old is aware of these sorts of body changes and discuss options that are available (which in this case are none for getting rid of them, though lotion can make them more comfortable if the skin is inflamed), going on about them has far more risks than benefits.
Do you all just accept your bodies with flaws and ignore them? I don't think you do!
It's one thing to be critical of ourselves, it's another to have someone who is meant to fully accept us keep pointing out our flaws over and over - even more so to a child displaying signs of being emotionally vulnerable.
When I was in an emotionally bad place as a young teen, I ended up shaving off my eyebrows because my mother would not stop commenting that I needed to deal with them and would make remarks when I didn't shave as often as she liked. I started to dress differently to fit her ideas: no one else seemed to have an issue with how I looked, I never thought I was attractive but even as someone who thought of myself as the ugly kid, it fucked with my head that I was less accepted in her home than I was anywhere else. I did a lot of things for my mother's approval but it was always out of my reach.
As an adult, I still remember her remarks. I don't shave or 'deal with' my eyebrows, I've little issue with my many stretch marks and scars - even the ones on my front teeth from repair work, I dress very differently now, but I still get the feeling sometimes that if my mother thinks this is ugly enough that I need to change, then I must really be ugly. I mean the expression is 'a face only a mother could love', if my own mother doesn't then...what does that mean about me?
Even the things she liked, like my being skinny, has at times itched my brain as the way I put on weight means I'm still 'bony' at my collar and hips and as those were her markers, I still see myself as bony even though I'm now a recommended weight for my height. Repeated focus on this sort of thing can distort how we see ourselves.
I also don't think we can compare stretch marks to thinks like acne and varicose veins which can cause pain and potentially dangerous infections. I get body acne and those little fuckers can hurt.