If the 12 year old is wetting the bed and will wear a diaper for bed wetting to keep their clothes dry and their bed dry then yes the 12 year old should wear a diaper for bed wetting.
However you should not force the 12 year old to wear a diaper for bed wetting if they do not want too.
You can buy Pull Ups for the 12 year old such as Goodnites underpants for bed wetting if they won't wear the diapers or buy Youth size diapers online.
Diapers would be more absorbent and keep the 12 year old's bed and clothes dry so they will be more comfortable.
My 10 year old wets the bed and yes he does wear some diapers at night but I didn't force him and just asked him if he would wear them.
He eventually decided to wear the diapers at night and wears the Parents Choice size 7 diapers which are not as babyish as other brands of baby diapers.