A kid is never too old to wear diapers while they sleep or whenever they need too.
If the kid is wetting the bed at night and needs to wear diapers they should wear the diapers if they will wear them until they stop wetting the bed.
Diapers are not just for babies or toddlers and even older kids wear diapers as well as teenagers and even adults wear diapers.
Some kids stop wetting the bed by age 8 to 10 years old and some kids continue to wet the bed until 17 or 18 years old.
Then again some people wet the bed all their lives and it's normal and if they need to wear diapers to keep their clothes and bed dry then they should.
Also if the child or teen, kid or adult needs to wear diapers for wetting in the day then they should also.
Diapers are made for everyone and not just babies.