Isn't Fuller too old to be wetting the bed by now?

With all that soda he drinks! It's possible.

Plus, he could have some kind of bladder infection or UTI ( which is mostly common in females but guys can get them too).

You can never be too old to wet the bed.

No.

There are actually some kids out there who have over-active bladders, or their bladder took longer to grow with their bodies as they matured. Fuller might be one of those kids. Plus, it was a gag they carried over to this film from the last movie.

Here's some fun trivia: the actor who played Fuller is actually Macaulay Culkin's real life little brother.