Do you teach your kids to wash their undies? how old you start teaching them?

Right now I wash all my kids clothes for them, but my oldest is only 5. They both know that their clothes have to go in the hamper, though - I won't tolerate them being scattered everywhere. I know some people wash their undergarments separately, but here, clothes just get thrown together. So if my kids know how to wash any load of laundry, they'll know how to wash underwear. When they reach high school, that's when I'll probably expect them to be doing their own laundry. They'll know before then how to do it, but won't be expected to do it themselves consistently until then. How can a person not know how to wash underwear? That's just crazy. Throw it in the washing machine. Add detergent. Push start. Not difficult.