Ducky comes from a land before time. That makes her older than time itself. Luckily, she was quite young when she came to this land and she has somehow managed to not age at all. Probably because she is older than time itself, and to prevent creating a rift in the space-time continuum, she will say a random number anytime someone asks how old she is.

Side note, her real name is Ducky but to avoid the space-time police, she uses the pseudonym, Duckie. Did I mention that she isn't the brightest skink in the pack?