If the 6 year old boy needs to wear diapers due to a medical condition such as a disability or being urinary or fecal incontinent then no the 6 year old boy is not too old to wear diapers.
If the 6 year old boy has no medical condition and is not incontinent or disabled in any way then yes the 6 year old boy is too old to wear diapers and needs to be potty trained.
However it can be hard to know if someone has a disability or to know if they are incontinent just by looking at them.
Diapers are made and sold for all ages including teens and adults so if the 6 year old requires diapers for either day or night time then they should wear them.
Some kids may wet the bed and need to wear diapers at night and that's okay.