Issues of age and eligibility are generally dealt with by commissioners and tournament directors.
As far as I know (and that isn't very far) there is no restriction on playing up. After all the division is known as 18u, meaning if you are 18 years old or younger, you qualify to play in the division.
lvtwft wrote:I am curious; I heard there was a limit of one age division you can play up. So, a 14U can not play 18U but a 14U can play 16U. Is this true?
Some leagues may have rules like that, but there's nothing in ASA Code. FWIW, there is a '96 on Tony Rico's Worth Firecrackers 18u team.