Depending on your waist size Luvs size 6 diapers should easily fit you as a 13 year old boy.
The Luvs size 6 diapers are pretty big for a baby diaper and they also have stretchy sides that stretch out pretty far.
I have a 32 inch waist and with the addition of an extra Luvs diaper tab from a used Luvs diaper I can make them stretch enough to fit me.
The Luvs size 6 diapers fit really close to my belly button and although I usually run some packing tape around the waist of the diaper they do fit pretty good.
I take the stretchy tabs off the side of the old Luvs diaper and then use a glue gun to take a Velcro Strip off another old diaper and glue it to the stretch tab.
Then I can reuse the stretchy tab on multiple Luvs diapers over and over and use them to help me fit into a Luvs diaper.
I mostly wear adult diapers but wearing Luvs diaper or other baby diaper gets me into the toddler mood.
Also the smell of Luvs scented diapers makes me feel more like a baby or toddler and I love the way it makes my house and even my car smell.
I keep a few Luvs diapers in my car and it makes the vehicle smell good as well.