Pampers Baby Dry size 7 are pretty stretchy and if you stretch out the pampers baby dry size 7 diapers and stretch out the tabs you could fit into the size 7 pampers baby dry diapers as a 13 year old boy.
I would also recommend peeling the cloth backing off which is easy to do as it makes the pampers diaper look much better and feel better as well.
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.