I'm having the same dilemma at the moment, my kittens are 9/10 months old (one is 3 weeks older than the other) and I keep thinking I should start switching them onto adult food. I'm thinking I will buy adult biscuits and mix them in with the kitten biscuits I currently have. Am also going to start buying sachets of adult food and try them out (mixing them in) to see which ones they like.
I thought about doing it at 8 months, asked the vet, and he said that was a bit soon, and that 10 months would be better