Pampers Swaddlers size 8 diapers will fit a 13 year old just fine as long as they are not overweight.
If the 13 year old is of average weight and has a skinny enough body and waist then the Pampers Swaddlers size 8 diapers should fit perfectly.
I actually saw a boy of about 12 years old today at Walmart wearing a Pampers Swaddlers size 8 diaper under his pants.
His shirt rode up and I could see the diaper and also his mom put 2 boxes of Pampers Swaddlers size 8 diapers in her shopping cart.
He didn't seem embarrassed at all and seemed very happy about the diapers.
Pampers Swaddlers size 8 will fit an adult if they are skinny enough.
I'm an adult and have a skinny waist and I can easily fit into a Pampers Swaddlers size 8 diaper without issues.
If you have larger than a 32 inch waist then I would say that you won't be able to fit into the Pampers Swaddlers size 8 diapers unless you modify them.
You could remove the Velcro strip and then cut it or tear it in half and glue one part of the Velcro strip to each side of the diaper to extend it.
I do this with size 7 diapers to make them fit.
If you have a waist under 32 inches then you should be able to easily fit into a size 8 diaper.
I would say that Pampers Swaddlers size 8 diapers should fit easily on people and kids and teens up to age 15 to 18 depending on how big of a waist they have.