I thought that in the Last Battle, Eustace and Jill said that it was only a year that had passed in England, but I could be wrong. As for the time that has passed in Narnia, there's no doubt it's been a very long time, as there are about seven generations between Rilian & Tirian.

Where does it say that there was a 7 years gap??

In the "official" timeline, which has been hand-written by Lewis and printed for the first time in Walter Hoopers "Past Watchful Dragons", which is the basis for all the Narnian timelines today.

In CS Lewis's timeline for Narnia 7 generations in Narnia has passed and 7 years on earth passed for Jill and Eustace. SC was in 1942 and LB takes place in 1949.

Textual evidence - info drawn from the text:

We know that there is one year between PC and VDT (summer holidays in VDT when Peter is preparing for a future big exam/set of exams, probably for entrance to university at age 16 or so [this supports the timeline book age of about 15].
Susan at a year younger is not going on with studies, and is able to visit America with their parents. VDT book ages for Lucy and Ed are 10 and 12.

Then there's a gap of a few months between VDT and SC - Jill admits several weeks into the new autumn term that "this term" Eustace has been a better person, and he tells her about what happened in his summer holidays (visit to Narnia at the start of the summer). These two are at boarding school, which usually doesn't/didn't begin much below age 11 when secondary schooling begins in England. This makes them either slightly younger than Lucy or about the same age - she had started right after PC.

By LB we are told that Jill and Eustace are the only ones left at school, and since the youngest they could leave then was 14, it has to be at least two to three years on. If Lucy was a similar age, we have to conclude that she didn't stay on to do higher education, and has left to do some other training or work.

The description of the seven friends of Narnia as seen by Tirian is interesting - Peter seems "hardly full grown" although king-like, and younger than himself, Edmund "the other youth", Lucy "a fair haired girl younger than either of these", and Eustace and Jill "younger still".
Tirian's age isn't established, and his view of the Pevensie brothers is hard to draw conclusions from. But Peter has to be somewhat older than the 16/17 he would be only one year after VDT. And Edmund has to be much older in the book than the 12 or 13 he is in VDT.

Finally, Polly says Susan "wasted all her school time wanting to be the age she is now" and will probably spend the rest of her life trying to stay that age - which has to be a young adult, maybe 20 but possibly as young as 18 or as old as 21. In VDT (and SC) Susan was 14 or 15, and if she is now at least 18, then LB is at least three years after SC.

Tirian's age isn't established

It sort of is. Lewis describes him between 20 and 25 years old.

My first thought is "oh, another discrepancy in the timeline." I hadn't noticed this one before.

VDT book ages for Lucy and Ed are 10 and 12.

Are they? In LWW it says that there is only a year's difference between Edmund and Lucy.

I always thought that Peter was over 20 in LB, but I have no real evidence for it.

The ages of Jill and Eustace in SC are 9 years (book) the age of them in LB are 16 (book) so that is seven years since.

The ages of Jill and Eustace in SC are 9 years (book) the age of them in LB are 16 (book) so that is seven years since.

Does it say that in the book or the timeline? I can't remember anything about their ages in the books, but I haven't read them in awhile.

I also seem to recall that there is a seven year gap, but I cannot for the life of me remember where I'd heard this.

Wow, seven years? I never imagined it as that many, I thought 2 or 3 at the most, but then again, I never really thought it through properly.
The thing with imagining it inside your head is that not everything has to make sense.

Does it say that in the book or the timeline? I can't remember anything about their ages in the books, but I haven't read them in awhile.

That is only in the timeline, hence why I opened this thread

One thing which would make it very hard to believe (at least for me) that if it really is meant to be seven years apart is that the behaviour of Jill and Eustace is very much the same as in the other books, which is nothing like I would imagine a typical 16 year old

So my theory is that either it really was a mistake on Walter Hoopers part while reading Lewis' handwritten timeline, or it is just a simple inconsistency on Lewis' own part (similar to the two swanwhites, or the date of VDT and WW2).

In my opinion I really don't care because Will Poulter will be in his 20's when LB even or if it gets here. So he wont be played out as an early teen. He's played out as a fifteen or fourteen year old in VDT already so to me the ages does not matter anymore.

I still believe in the timeline though. It seems as if London in LB is being played out as a little bit more safe, the ages of the Pevensie's seem to be 6-7 years older then how they last appeared. So truth is Jill/Eustace are the correct age of 16 in the book.

The only thing that has always confused me about this is that Eustace tells Tirian that it hs been "more than a year" since they were in Narnia. Now this is maddenly vague. I could buy two or three years based on this comment but who labels seven years as simply "more than a year" when describing it in conversation? And if it had been three years, why wouldn't he just say that? But if it had been only a little over a year, Lucy would still be in school.

So my theory is that either it really was a mistake on Walter Hoopers part while reading Lewis' handwritten timeline, or it is just a simple inconsistency on Lewis' own part (similar to the two swanwhites, or the date of VDT and WW2).

What's the inconsistency in that date?