Please don't flame me for what I'm about to say - it is honest, and it is our decision. If it makes you uncomfortable - please don't read...
Our kids birthdays are in September, and we have taken a big December trip every year. As it turned out, at some points in these vacations our kids were 3 and 3 months, respectively, when we went. We did not get them a ticket and we were never asked at all.
First off - why??? We go to Disney about 3 or 4 times a year and all and all I can attest that Disney gets plenty of dough from us. I didn't particularly feel guilty, because the rest of us had annual passes, and the youngest kid (3 years 3 months) really didn't ride anything other than an occasional Small World, Carrousel or watched a parade. Seriously - our kids were not adventurous at that age, so we were actually spending more $$ when you think of counter service costs, souveniers, etc (which we definitely bought.) We will continue to go there multiple times a year for the rest of our lives, so over time that one trip will really come out in the wash. Plus - since we had annual passes, we would have either had to buy a totally new annual pass for the just turned 3 kid (with new expiration dates different from the family) or park hopper that would just sit, because we would be buying a new AP the following year. And really when you buy an annual pass it is not like Disney gets a buck for each time you enter. Anyway, I can justify it all day long - but the bottom line is that either you feel comfortable doing it or you don't. There are certainly excellent arguments for both, so I would say do what is in your comfort zone and what, all things considered, would be reasonable for everyone involved.
That said - if you want to do it, my best advice is to try to keep the kid from talking. Size won't distinguish a 2, 3 or 4 year old necessarily, but verbal ability will. I wasn't the least bit worried when my now 5 year old was 3 years 3 months, because he was sitting in the double stroller next to his sister who was 2 years younger, and therefore he looked younger. However, when she was 3 years 3 months, she could really talk well... plus he was huge so I wasn't quite as confident.
Anyway - remember that 3 is generally the universal age when most kids can tell you how old they are, so if you are worried try to make sure you just go through quickly so CM won't talk to you or child.
However, what we found is - by and large CMs don't care. The wait staff doesn't care at a buffet, because they are looking for a better tip. They aren't going to cross examine you - and for that reason we tipped VERY well if we ever took her to a buffet (she ate like next to nothing.) The front guard folks generally don't care either. Just carry the child in, or put child in a stroller and give them something to eat/drink or a map to look at so they won't talk.
I wouldn't push it past a couple months though.
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