Pampers Swaddlers size 8 diapers should fit you pretty good at 13 years old if you're not overweight and are of average weight.
I could've easily fit into a Pampers Swaddlers size 8 diaper when I was 13 years old as I was very skinny and wish I had those diapers at that age.
Now I'm no longer 13 years old and have a bigger waist and I can pretty easily fit into a Pampers Swaddlers size 8.
I can even squeeze into a size 7 diaper but the size 8 Pampers Swaddlers fit better.
So I would say yes you could fit into a pampers swaddlers size 8 diaper at 13 years old if you're skinny enough.
I would give them a try as you certainly can fit into the pampers swaddlers size 8 even if you have to modify them slightly.
But I would bet that you would fit into them without modification.