So I think it's safe to assume that, Littlefoot and Cera are about 7 or 8 in the original, and Ducky about 6. Both of them age probably no more than 2-3 years in the course of the series, if at all.
Than we have Spike and Petrie to consider. Petrie's hatching is never shown, so we don't know how much younger or older he is. In 1, 2 and 4, judging by his voice, I would consider him to be a tad older, maybe around 10. However, in 3, 5, and all the movies that follow, his voice makes me put him with Littlefoot and Cera at about 8. So it's confusing.
Than we have Spike to consider. He was born by the time the gang had reached their "permanent" ages. So he doesn't live through the timeskip that occurs early in the original movie, yet somehow is as big as Littlefoot and Cera. A long time ago, when I first joined GOF, I made a whole topic about this, btw.
Than we have Chomper to consider. He's even younger than Spike, considering he was born a movie later. Yet by movie 5, he seems to be about Ducky's age, while Ducky herself hasn't aged.
So I'm just going to assume that the characters stop aren't allowed to age much.
Here's what I've calculated:
Movie 1
Littlefoot: 7
Cera: 7
Ducky: 6 (I'm gonna assume her hatching wasn't shown chronologically in movie 1)
Petrie: 7
Spike: 0
Movie 2
Littlefoot: 7
Cera: 7
Ducky: 6
Petrie: 7
Spike: 6 (6 year time gap between original and movie 2 release taken into account)
They remain that way until 5, in which Cera turn 8, making her the oldest.
Then, in 7, Ducky finally ages a year (her vocal evolution happens here) putting everyone in the gang except Spike and Cera at 7.
Than, in maybe 11, I'd have her, Cera and Spike age a year.
So than we have this:
Movies 11-13 + TV series:
Littlefoot: 7
Cera: 9
Ducky: 8
Petrie: 7
Spike: 7
Just my theory!