At my school (small private high school), the 2 competing impulses are to make the letter worth something but also similar to other varsity sports. Every member of the varsity football and basketball team letters, but making bball is hard (plenty of good kids get cut). Football is no-cut at our school, so 35-40 boys letter. But varsity is grades 10 and up, and only the very rare freshman makes the team.
For XC, we bridge that gap by lettering seniors, juniors, and returning sophs based on participation (show up for everything). But to letter early (middle school, 9th, or as a 10th-grade rookie) requires running top 7 in half or more of our meets. Coaches can fudge a little if it's close.
For track, the same thing works for seniority, but you can also letter by scoring 20+ points on the season and/or posting a qualifying time for our state meet. Those almost always correlate. Sometimes a kid who really ought to letter but doesn't quite have the times can sneak in as a weak leg on a relay.
It's really a pain--you don't want the kid running 27 minutes to letter, but then you see the skinny sophomore who never gets off the pine in football or soccer in his letter jacket. But place alone won't work; our 22nd boy would have scored for several teams at our state meet.