Beginners should start with 6-8 miles and progress to 10 then eventually 12.
Above Average/experience would start at about 10 and progress up to 16-18. 20 on their heaviest week.
Elite kids can handle 20-25 if necessary. They only race two miles and they're still in JH. You don't want to burn them out.
Long runs would usually be 4 miles for most runners. Some kids can handle 5. There are probably kids that could handle 6, especially the top athletes, but most of the time it's just not necessary given they only race 2 miles at a time.