Yes it's normal for a 15 year old boy to wear diapers that are made for babies and toddlers and even to wear adult sized diapers.
Diapers are not just for actual babies and toddlers.
Diapers are made in all sizes and if you prefer to wear diapers whether baby diapers, toddler diapers, adult diapers etc you can do so.
I used to wear baby diapers as a teenager and still do now as an adult.
I now also buy adult disposable diapers as well.
But when I'm in the mood for playing toddler I want to put on an actual baby diaper such as Luvs size 6 diapers or pampers size 7 diapers.
After using my adult sized disposable diapers I cut off the side wings and then glue them to the sides of the back of the baby diaper.
Then I take the Velcro tabs of the baby diaper and also glue them to the larger wings that I extended the baby diapers with.
That's a way to make the baby diapers such as Luvs and Pampers Cruisers size 7 diapers fit my 32 inch waist.
Baby diapers feel even better to me than any adult diaper does.