If the 10 year old child has no medical issue or is disabled in any way then they should be potty trained and not be wearing diapers at least during the day.
However sometimes 10 year olds can be incontinent or be disabled and need to wear diapers or they have autism or something else.
If the 10 year old requires the use of diapers then there's nothing wrong with that and they should wear diapers or the 10 year old may wet the bed and need diapers during the night as well.
Diapers are made and sold for all ages and if they need the diapers they should wear them.
Some kids may also like to regress and wear diapers because they like the diapers and like playing as a baby and if so they should be allowed to do so.
Some 10 year olds may just like to wear diapers because it's something they enjoy and then they grow up to be diaper lovers and there's many of them out there.