Pardon me while I digress...
When DD was 3 months old it did not evern occur to me to bring age documentation because she was so obviously under 2 (and I didn't recall needing it for my older DD when she was lap-age - they may have changed the rules here in the past 3-4 years or so between my kids...) Anyway, the check in agent -- I believe her name was Snotty McBiotchy pants -- threatened to not let me board the plane or require me to buy the baby a seat because I didn't have documentation. Thankfully, Snotty McB's grinch-sized heart must have gotten a small burst of extra blood because she eventually relented and let me through. Might have had something to do with the check in agent next to her who looked at me, then my DD, then Snotty McB, and said "you're kidding me, Sheryl, right? Don't be a jerk."
Fast forward 2 months when I was flying with the now 5-month old...I have my trusty documentation (everything but a dispensation from the Pope) and hand it to the check-in agent who says "you don't need that, hon - she's clearly under two!"
Go figure.
Jackie