If you buy, but don't use a ticket then it doesn't expire, and can be used at any point in the future.

A quick note about the child tickets ... if you buy a child ticket, and the no-expiration and the child "ages" out of the ticket (turns 10) after only having used part of the ticket, you can upgrade it to an adult ticket for free (for the same bigger child to use). Sometimes this helps a bit with the cost.

Otherwise, I know many people who buy tickets in advance, since the cost will only go up!