Well, the Simpsons have been going on for what, 17-18 years? Bart and Lisa haven't aged a day and the show features current events too.
The Simpsons even contains a number of jokes referring to this exact fact about the series.
For example:
Lisa: "I feel like I've been wearing this same red dress forever!"
The producers use and abuse this fact for the series, and have simply made it into one of the elements of the series.
LBT on the other hand refers to aging on a number of occasions
Cera: "It's just like waiting for your hatchday! It never seems to get any closer." (LBT 5),
Cera: "I'm practically all grown-up!" (LBT 9)
or
Littlefoot: "I'm bigger now." (end of LBT 11)
My largest concern is the comment made at the end of LBT 11 ("I'm bigger now.")
I realize that the producers added this line in to conclude the film, but I"m a little concerned about what that shows about the developing of the characters. They have obviously aged since the beginning of LBT 11, but knowing the producers, I suspect this fact will be completely forgotten for future films.

I realize that "The Simpsons" also refers to birthday parties and other aging elements, but I'm talking mainly of LBT here, not The Simpsons.
If I had to guess the ages of the LBT characers (in human terms), Here's what I'm going with:
LBT 2: 7-8 yrs old
LBT 5 and later: 8 yrs old (slightly older than LBT 2 - 4)
end of LBT 11: 9 yrs old