Gentle Steps Size 7 diapers are pretty big and I'm almost 33 years old and can almost fit into the Gentle Steps Size 7 diapers although they are not a perfect fit.
The Gentle Steps Size 7 diapers do fit by stretching the tabs out a bit and I use some packing tape to secure it around me.
Gentle Steps Size 7 diapers also are very absorbent and hold a lot of pee and are really great diapers that swell up nicely.
I love the SAP in the Gentle Steps Size 7 diapers from dollar general and they are a great diaper for the price.
I have a 32 inch waist so if your waist is smaller than 32 inches which mine was when I was 15 then the Gentle Steps size 7 diapers should easily fit you as a 15 year old.
They're cheap and worth a try and if you cannot fit perfectly into them then take the Velcro strip off and cut the Velcro strip in half and then glue one on each side of the diaper which is what I do.
Then they can fit and you can also add some packing tape to it if you want.
They only cost $5.00 per pack for 20 diapers which is cheap.