Moyra Charlton - amazingly only around 11 or 12 when she wrote her first book Tally Ho.
It is difficult to come up with a reason for this precocity. One suggestion is that children matured more quickly then. But if we examine the ages of some of the non-pony female authors of the same period, such as Enid Blyton, E. M. Brent-Dyer, Noel Streatfield, Richmal Crompton and Lorna Hill, we find that all of these authors were in their 20s to 40s when they began writing novels. It does in fact seem a trend amongst horsy rather than general authors.
It would be nice to come up with a theory to neatly explain the precocity (and if anyone has any suggestions please let me know!) However, whatever the reason, let us be grateful that our favourite authors did not consign their early works to a dusty attic along with their old dolls and teddies, but allowed the rest of us to share their youthful writings.