Once upon a time, many moons ago, I ran a full season of fellrunning as an 18yo that included several mediums (Fiendsdale, Kinder Downfall, Coniston, Fairfield) and a couple of Longs (Winter Hill, Three Shires). I wasn't particularly good at running at the time and not particularly well trained for them. Physically though, no problem whatsoever, got round them all comfortably and with enjoyment. 16 yos are allowed to run marathons aren't they?

However, the crucial thing is that I was lucky with the weather for most of them and only got clag once (Fairfield, which was an championship so you just followed people anyway). I can read a map. But if something had gone wrong that required mountain experience or judgement (sometimes its just about knowing when to stop) I'd have been woefully underprepared (and I was generally fairly sensible and well used to fellwalking with my family etc.)

I'd say that there's no problem with youngsters running mediums at times of year when the weather is likely to be decent and on courses where the consequences of doing something wrong are less significant. This could be at the organisers discretion. But I wouldn't send one round Borrowdale.