Yes 8 year olds can wear diapers.
Although an 8 year old should be potty trained during the day if they don't have any health issues an 8 year old can wear diapers.
They make diapers for all ages of kids, teens and even adults.
If someone of any age needs to wear diapers then they can and should wear the diapers.
Some 8 year olds and older kids are incontinent or have autism etc that makes it difficult for them to use the potty or potty train.
So then those kids need to wear diapers.
Also some kids and 8 year olds wet the bed and need to wear diapers as well for the bed wetting.
If the 8 year old needs diapers and needs to wear diapers then they should do so.