Don't know about that, when i started out I was only doing speedwork no longer stuff, admitedly this as a junior so not sure how well this aplies to everyone else but still doing maybe 25% to 1/3 of milage as speedwork and am fine. I think that it in fact a great way to start as long as done sensibly, really gives you an edge in races only having to do a fraction of max pace when compared to others. Start on a grass track if possible doing maybe 6 to 8 X 300 (even 150's are fine) a decent pace with a rest that leaves you able to do all of them, know my club does a speed session on a local grass track often.
Really I think it whatever you're happy with, if you don't want to risk it don't, if you want to give it a go then don't feel that you're starting it to early, as long as you do it in a controlled way.
