The gap between the first two stories is about a year. 

When they come out of the wardrobe in LWW no time has passed. It's the summer holidays. 

A year later, they're at a small railway station on their way to their boarding schools (boys to one and girls to another). It's Lucy's first time.  This time they don't get older, as this visit is only a few weeks or days. They don't revert to their adult Narnian status.

The third visit is the start of the following summer holidays. There's no age change at all.  

The fourth book is a few months later, soon after the new school year has started. It's autumn, and interestingly enough, it's Autumn in Narnia too!  The two children don't change age.

Posted by: @kj7rrv

Does this mean that a character's age is tied to time in their home world, and their age in other worlds "resets" each time they go home?

Yeah, that's the impression I get. The main characters technically grow younger whenever they get back home in every book, but The Lion, the Witch and the Wardrobe is the only one that mentions because it's the only one in which they're gone long enough to visibly age. 

Thank you! It seems, then, that there is some kind of inherent connection between a character and the world in which they are born that remains no matter how long they are in another world? I haven't read the whole series for a long time (currently re-reading the series, now part-way through PC), but I don't remember any character (including the Pevensies, as Col Klink noted) spending a long time in another world then going back "home" except in LWW. (Jadis is from Charn, but never goes back to Charn after arriving in Narnia.)

Polly and Digory only travel for a few days back and forth by the Wood Between the Worlds. In PC, I'd reckon the children from our world stay for a few weeks (or shorter) before going home. In VDT and SC it could be a matter of months rather than weeks. But LWW is the only book where anyone stays for more than a year and then goes back home.