Altar servers usually start fairly young 3rd grade or so. Of course, they are trained, are doing the same thing every time, don’t have to read or sing, and can receive direction from the priest on the spot if necessary.
A child serving as an usher with a parent, should still probably be 10 or older to be effective. Our parish stopped allowing children to serve as ushers after part of a collection was stolen during Mass. It was stolen by an adult, but the baskets were taken from the children ushers who didn’t realize he wasn’t also an usher.
Readers and cantors should receive lots of training (adults also) and practice the specific readings/music for the Mass. Young children should just read at children’s Masses. Older teens can be at regular Masses. We had a teen reader (lector) at Mass today who was an excellent reader, but was obviously nervous and unsure of where to be during the psalm or when to come back up for the prayers of the faithful. That is why kids should go through the same training adults do.
As for cantors, I am less inclined for this role to go to a child/teen because so much preparation in necessary and most kids just won’t put in the time needed.