Early teens would suggest the girl is aged 13 to 15? Isn't that a bit old to be spending her days bouncing on a child's trampoline?
You'd think that, but I suppose there would be no harm if she enjoyed bouncing on it at her age - although I find it rather odd that they've only just got it, rather than, say, her having had it since she was 6 and just never having stopped wanting to keep using it.
That might even be a bone of contention - if her mum (or another family member) got it for her for a birthday present simply because it was very cheap or even off FreeCycle rather than because it was deemed a greatly desired, lovingly-sourced and age-appropriate gift.
She wanted something expensive and more grown-up, but mum/dad/gran/uncle found a very cheap trampoline, thought "that'll do - it's a generic kids' thing!"; her mum knows it's not really what she wanted, but feels guilty so keeps saying "Go and play on your trampoline - don't you like your new present?" The girl doesn't like to say "No, I don't actually - it sucks: I'm 15!" but of course, she's too old to enjoy just bouncing, however staring and people-watching are a bit more like it.
Having postulated that, though, teenagers can be extremely fickle with what's 'cool' or what's 'for babies'. It never ceases to amaze me (well, not right now whilst it's locked for COVID) how the teenagers like to congregate in our local kiddies' play park. I like to think they pre-arrange it and exchange texts about 'hangin' out 'n' chillin' wid ma bredren on da climbin' frame, innit' 
They posture and swagger and really give off 'so very cool' vibes - whilst sitting on a seesaw designed for a 6yo!!! I saw one girl stuck in a baby swing once - it was hilarious watching her street-credibility plummeting southwards with every second of struggling to extricate herself 
It's more annoying than funny, really, as they lurk and swear and make it a generally unwelcoming atmosphere for the little ones for whom it's actually intended.