Teams can substitute runners with replacements from their team that are of the same age or younger(as runners can run up but not down in the relays).
Distances for ages 8 and under are as follows-
1st,2nd,3rd,4th, and 5th leg run 1.3 KM each
Distance for ages 9 and older are
1st, 3rd, 5th run 1.8 KM each
2nd and 4th run 1.3 KM each