Those rules are bollocks if you ask me anyways, if you're under 10 you can get a TY80, (We didn't bother with that though, just stuck my brother on a TY250, he couldn't even start it until he was 12, and then he had to stand on something with his left leg ), but if you're say 13, what have you got? Either a Fantic 125 if you can actually find one, or an old knackered GasGas... (The only one's I've seen for around £600 have always needed something, usually engine rebuilds and shock linkages).
Then if you're 16 and wanting to start comps, you've got nothing because unless you can afford a modern 125, they're all pretty gutless! (I'm basing that on my mates old GasGas 200 Pro from 2004 being utterly nothingless when I rode it, when I was 14 and about 8 stone!).