Well from my understanding the space between the eyes rule still applies, but as they get older they have a greater ability to eat the adults. Mine also chews his food rather well and sometimes bites them in half, which is kinda gross. All you can do is feed a small amount to see if your dragon can handle them properly. I've also heard of people feeding adults around 12inches in length, i personally waited till he was about 16-17 before i tried to feed him anything remotely big.
I started off small by just feeding 1 adult roach, then his normal diet. i waited a day to make sure his poop was fine and that there wasnt any issues with digestion. I also did the same for super worms, start small and slowly build.