Yes a 14 year old can wear diapers.
Diapers are made for all ages and you can wear diapers at any age even if you want to wear the diapers for fun.
If you need diapers at 14 years old or you just want to wear the diapers at 14 years old for fun then you can and should wear the diaper.
Diapers are much more comfortable than boxer shorts or regular underwear.
I gave up my underwear over 8 years ago now to wear diapers full time.
I'm almost 33 years old now and I wear diapers all the time and no longer use the toilet for either poop or pee.
Diapers are much better to pee and poop in than the toilet and I feel more comfortable wearing diapers than wearing underwear.
Also it can sometimes be hard to find a toilet in public and when I need to pee or poop I just use my diaper instead.
Also if I'm outside doing something and need to pee or poop I don't have to run inside the house to use the toilet and can just use my diaper and continue doing what I'm doing.
There's a lot of 14 year olds and above who like to wear diapers or need to wear diapers and it's okay to do so.