My son is now 6, but everyone thought he was 4 when he was only 2 1/2. We went to Disney and I forgot to bring his birth cert. We got questioned at the gate every time, but they let him in with doubtful looks on their face. Then one castmember did not ask and I told them how old he was and that everyone else was questioning me on his age. The CM said if they really doubt, they will usually ask the child because it is rare the child will give the wrong age. After I thought about it, several had casually asked him how old he was.
But when he was 3, we went out to dinner at Maggiano's and were doing the family style dinner - you pay for your kid when they are 5. Again - people always thought he was older. I would have him at the grocery store and people would actually ask me why he wasn't in school and their jaws would drop when I told them his age. I told the waiter he was only 3 and my son says, "No I'm not, I'm FIVE!" I thought I was going to die! They didn't make us pay for him though.
We just got back from our trip and my DD2 looks 2 to me, but people always ask if she is 3 or 4. Don't know why, she still wears 18 mos clothes. We were asked once how old she was at Animal Kingdom and they let her in with no problem.