Littlefoot on it also confirmed it by saying she is the oldest saying... "but that's you" and her reply with a grin was "I know" I would guess they about young teen ages roughly, not very sure. No one really knew what the child hood length was for a dinosaur.

all we know it could of been about 18 years or something growing a little each year getting taller but the gang on Land Before Time seem to not even grow that big maybe a little bit but all the adventures that they had and different phases saying things like in Land Before Time 10 Bron stats he was looking for Littlefoot for years and spoke of the egg which is Littlefoot hatched. he could be saying 3 or 4 years no way to really tell... but from the point of view in Land Before Time. at the time of them hatching it was almost like a year or maybe longer after they hatched that they could talk to their parents and start to learn about things in the world

There is so many different conflicts to this it hard to tell what about their ages.

-Threehorn

I would like to imagine that Cera lied to the rest of the gang of how she is the oldest. With her ego and all, I would not be one bit surprised if Cera did lie.

I could imagine that too. However, it is generally difficult for me to imagine that the dinosaurs would know exactly how old they are. While I suppose they know how many cold times, times of the changing treestars etc. they have seen I don't think they would even know the numbers to say "I'm 2354 days old".
I suppose that at least the settled dinosaurs may have a relatively accurate "calendar" by observing where exactly the sun rises and sets (the outlines of the Great Wall against the horizon would probably provide sufficient landmarks so the dinosaurs could see how the point where the sun is rising shifts gradually from the left of a given summit to the right of it or the other way round. Nonetheless I don't think that the dinosaurs while searching for the Great Valley had the means or the time to even bother about this which is why I don't believe any of the characters knows exactly the number of days since he or she hatched. In case they hatched just a few hours apart (which is the impression I get from the original movie) it would be even more difficult to decide who was first.
One point that may work against this thesis of mine is Cera's mentioning of waiting for "hatchday" in LBT 5. Yet I suppose that her hatchday may have been "settled" once the dinosaur were in the Great Valley and had the time to bother about time measuring. Maybe a day was picked randomly (the day when the bright circle rises exactly above the top of summit XY) in a time of the year when the climate was similar to that on the day of Cera's birth.

While I suppose they know how many cold times, times of the changing treestars etc. they have seen I don't think they would even know the numbers to say "I'm 2354 days old".

Do they even have "cold times"? Judging from the vegetation,  the Great Valley looks tropical/subtropical, with a lot of jungle, and while temperature does become more mild during wintertime in the subtropics, it rarely feels "cold" unless one lives close to the ocean (coastal locations can receive plenty of wind).

As for the main question...I would think Littlefoot's the oldest, since he seems more mature than his friends.

Just because he is more mature doesn't mean he is older. Besides, I'd say Ducky is the oldest because she is the first one we see hatch. Cera could have lied when she said she was the oldest. But maybe that wasn't Ducky we saw hatching. It may have just been some random swimmer.

I agree with you on this one, F-14.  

Regarding the situation in LBT 5 where the Gang agrees that Cera is the oldest:
Let us remember a similar situation in LBT 7, where Cera is arguing about always being at the back of the line while crossing land gaps.  The Gang agrees with Cera saying "she do have a point" for example, as if the Gang has forgotten the details of their past adventures.  If we look back to earlier films, we clearly see that this whole story about Cera being last in line is complete bogus.  

I'm saying, if such a false bit of information can be protrayed in LBT 7, why couldn't the same thing have occurred regarding the age discussion in LBT 5?  I'm going with this:
The discussion in LBT 5 is false, incorrect information, as the series has a reputation for making such errors (sadly ).  

As for who IS the oldest?  The first LBT film shows a swimmer being born, who is presumably Ducky.  The fact is, we don't know for sure whether this is Ducky; this scene may be interpretted in a way to show various families bearing children, of all different walks of life, not necessarily ever destined to work together.  This shows the vastness of the species headed away from this land, to the Great Valley (or other hospitable land).

It is probably a safe assumption that our little swimmer from the first scene is Ducky; even her first word, "Momma", is spoken in a typical Ducky voice.  However if we look towards the later sequels, namely LBT 8, we see that Ducky's voice is very, Very similar to her siblings.  I have reason to believe that Aria's quotes were reused for the voices of Ducky's siblings in LBT 8.  My point?  Perhaps all baby swimmers sound so similar that telling them apart by voice may be nearly impossible.  We cannot Prove that this is Ducky in the first scene, I don't think.  If someone can correct me, that would be fantastic. B)

For all intents and purposes, everyone in the Gang is the same age, except our friend Spike.  However when it comes down to technicalities....

Spike is clearly the youngest, as the rest of the Gang finds him when he's still an egg.

Petrie is unable to fly when the gang meets up with him, therefore I believe him to be one of the younger members of the Gang.  

Ducky is a fairly capable swimmer, therefore she can't be especially young (can she?)  I'm saying she's considerably older than Spike.  However she has grown very little (compare her size to the size of one of her eggs in LBT 2.  She has hardly grown at all).

Littlefoot and Cera both grew a substantial amount from when they were first seen, therefore, I put them among the older members of the Gang.  Unless the discussion in LBT 5 is indeed true, we have no way of figuring out which of these two is older, I don't think.

Based on this information, I'm saying, from Oldest to Youngest:
Littlefoot/Cera
Ducky
Petrie
Spike

I've engaged in some research of my own but I"m sad to say that It is impossible to judge age in my opinion.  The only thing we can truely go on is their size(which is never changing).  It is also impossible to judge exact size because we had no idea what the size of the rocks and trees are.  It remains a mystery to me.

How many winters do you suppose they've went through that we know of?

That is not a valid indication either, I"m afraid.  The weather was significantly different back then.

But we have no idea how long the cold time's last.  For all we know, they could last a year themselves!

It seems that they're only growing in maturity.  Their size has only fluxuated slightly through the series.  Littlefoot was smaller in LBT 10 than in LBT 2!  It's just too impossible to get an accurate assumption.

He was smaller in a couple of places.  I compared his size to his granfather's head.  I should post pics of that sometime...