Really, it's hard to say how much time has passed between the 14 movies.
In the first movie, the time from Littlefoot's birth to the very next scene is supposed to be 5 years, at least according to a book about it. From there, it's about a week before they arrive at the valley. From there, the only time skip before 2 is answering the question of how long it would take for Ducky's mother to have new eggs. By the end of the movie, they all hatch.
3 could have happened not long after 2. In 3, fire happens, burning a good chunk of the valley. It's recovered by 4. Then comes 5 where all of the food is eaten by bugs, forcing them to leave until it's regrown. 6 has Dana and Dinah, who are somehow Cera's niblets and can talk. 7 has no indication as to when it happens after 6. 8 has the first cold time. 9 has a flood and there's no indication of it ever happening by 10.
11 has another problem. Tria. Topsy and Tria get together and have an egg that hatches by 12. Multiple days pass, leading up to The Great Day of the Flyers. 13 could have happened immediately after. (Though some question if it ever did happen.) Then comes the TV Series, which has it's own problem of time discrepancies. By 14, Ducky's mother has more eggs. Those hatch at the beginning of the movie.
It has definitely been a few years at least, but the kids haven't aged a day. This is a phenomenon known as "Comic Book Time". It's where the main character never ages, but the rest of the world, including those they know, plus those that are born in that time, do age. It works better with adults in late 20s/early 30s. At least there you can get away with it. However, kids are a different story. When they are the main focus, they never grow older, even though they should. Especially with the fact they get a sibling or something.
The odd phenomenon has happened in a lot of Animated Series. Rugrats had Tommy be 1 1/2 years old, get a brother, and he's STILL 1 1/2 years old. Maggie was 1. Apu gets married and has 8 kids. Guess what? Maggie is still 1. Then we have Family Guy who had a woman be 8 months pregnant for quite a few years before giving birth. (Yes, even multiple Christmases happened and she's STILL 8 months pregnant.) Oh yeah, and Ash from Pokemon is STILL 10, despite the fact that a few years have passed. (Some find this VERY hard to accept.)
So yes, the gang are also victims of Comic Book Time. They haven't aged a day while everyone else has. We may as well accept that they won't age until the writers say they do. There's no scientific explanation for it. They won't age until the writers say so.
