Stretch marks on skinny 13yr old DS

[Minzee]

Ds, only just 13, is a skinny kid. You could barely pinch an inch anywhere on him. Tonight I (accidentally, obvs) caught sight of his bare bottom exiting the bathroom and I saw that he's got really angry, new looking purple stretch marks all over his bum. Just contained to his bum and hips. He prances around in his pants daily so I would definitely have seen if they extended past his underwear lines.
He's my oldest so everything with him is new to me, is this common? I can't remember the last time I've seen him naked but I'm guessing the angry purple-ness means they're fairly new.
Don't know what I'm asking, or what I could do! Is it a marker for anything I need to look out for? I hadn't really thought of him "growing up" yet as both myself and husband were really quite late to the puberty party

Stretchmarks on teenagers are a thing & quite normal, nothing to worry about. I didn't realise until I noticed them on my eldest son's back, really vivid looking. He'd have been about 14 & having a growth spurt. Youngest DS got them too. They fade, both lads are grown up now & you can hardly see them.

My ds was skinny and had these across his back. He showed the nurse, whilst there about something else, and she said it’d due to growing. His have now disappeared. At the time they were very noticeable.

There are things you can do while they are still new. Creams, oils etc and I read about a derma roller I think it was called.

My ds's have faded this summer. You wouldn't notice them. Last year they were worryingly obvious in swim shorts.

My DS’s back is covered in them. They look like he has been clawed by a tiger! They first appeared when he was about 14 when he went from being a bit of a chunky lad to suddenly being like a beanstalk following a growth spurt. He is nearly 20 now and they have never faded but don’t seem to bother him.

I had them on my breasts as a teenager. Totally self conscious about them. You wouldn't know now.

My 17 DS has loads of stretch marks on his back. He is an athlete and incredibly fit and lean - absolutely no fat on him, so I was shocked when I saw them , but I guess it him growing faster than his skin is able to cope with. I'm sure they will fade and no one will notice them in future

I remember when my younger brother developed these on his back. He was always lean, too, but I guess had suddenly grown taller and the skin had stretched. At first glance I thought someone had been whipping him - they were so angry and red, quite alarming to look at. Now they're very faint and silvery, you wouldn't really know they were there. I think it's normal.