Gizzy younger than I thought

I posted not long ago telling you that I had put Giz on adult food but that he was being sick. So I started indroducing felix adult as he was on felix kitten food to see if that settled his stomach.
It seemed to work but yesterday he was sick (only ever a tiny amount). He has always acted the same, mad as a hatter!
I rang my collegue to confirm the date Giz was born and it turns out it was May, so he is only 9months old, and I thought he was nearly 11months old
Is he too young for me to have introduced him to adult food?
I have put him back on kitten food and he's not been sick today.
I feel really guilty for not checking his age before I introduced adult food. Could I have damaged him? I'm so worried and guilty

I don't think you have damaged him in any way. I think it's good that he's back on kitten food. I'm sure he will be fine. When you switch him to adult food in 3 months it is best to do it gradually. Mix about 25% of the new food with the kitten food for a week and then gradually increase the amount of adult food each week until that's all he is eating..

Thanks for the advice

You won't have damaged him!

I hope that with the next transition, things go a lot more smoothly for you!