Of course you can wear the PC Stock. But word of warning... the official PC stock pin is really blunt. Despite tack checking and PC committee etc experience for over 10 years (well over ahemm....) I haven't seen any age restrictions for stocks - but you do tend not to see them on the very little's and they become more common as the rider gets older. The only neckwear with an age restriction is the associates tie. My daughter has worn a PC stock rather than a tie to compete and hunt since they were introduced - so I suppose she would have been 11 or so back then. We do still use a tie for Quiz finals. (Although thinking about it a stock for H&P care finals...so not much logic there.) And for some rallies. Just get a plain stock pin that actually sticks....
From memory, I think my daughter wore the PC tie all the time she was riding a pony, but started wearing a stock when she went onto a horse at the age of 13. But there seems to be no fixed PC regulation about it.
You're supposed to wear the PC stock once you have your B test I believe. There are two problems - one is the PC stock pin is blunt as mentioned above (this can be remedied by using sandpaper to sharpen it up) and the other one is that from a distance it can look as if it is white/plain. So the photos of Teen 2 in her tweed make her look as if she is wearing an incorrect stock.