[I'll preface all this with where I'm getting the time gaps between the books from: LWW is in the summer; PC takes place 12-15 months later, in the fall; VDT is at the end of that school year, say ten months; SC takes place a few months later, in the fall. I don't think we have any real solid evidence for LB, but Lucy is finished with school, so this is probably seven years after LWW at the least. At any rate is has to be set after 1948, because they reference British Railways.]
Best guesses:
Lucy is ten in LWW, and turns eleven after they return to England. She spends the next year with the Professor, but turns twelve just before PC and her first year of boarding school. She's still twelve for VDT (in fact, her birthday would be on the boat), but she turns thirteen when they come back, just before the start of SC and her second year at school. She leaves school just before her eighteenth birthday, and LB is sometime after that, when she is seventeen at the earliest.
Edmund is an old eleven or youngish twelve for LWW; he starts boarding school when they get back. He is thirteen for PC and his second year at school. If he's young for his year he's still thirteen in VDT, but he may have already turned fourteen. He's fourteen and in third form for SC, and at least eighteen for LB, by which time he's been out of school (and at university?) for a year.
Susan is probably thirteen or fourteen in LWW, depending on how much older she is than Edmund, which means she's about fifteen or sixteen in VDT when she goes to America. (Personally I lean toward fourteen and sixteen respectively.) That puts her around twenty at the earliest for LB.
Peter is probably studying for his HSC in VDT, which means he's probably seventeen-going-on-eighteen for VDT and SC. Working backwards, that means he was sixteen/seventeen in PC and fifteenish in LWW, and of course twenty-two or so in LB.
This makes Lucy and Peter a bit older than I thought, but I think it fits. It gets more complicated if you consider that Lucy is done with school in LB but Eustace is not, even though SC is her second year or boarding school and (at least) his second year at Experiment House, but I'm just going to assume that Experiment House gets them at a much younger age--that also means Eustace and Jill can be several years younger than Lucy. More detailed and rambling explanations where I worked things out are below!
Lucy's first year of boarding school is in PC. She's probably about twelve then, for first form, which makes her ten/eleven during LWW (depending on her birthday). If she's young for her year (say late summer or fall birthday), she's going to be ten/eleven in LWW, eleven/twelve in PC, twelve in VDT, and twelve/thirteen in SC. If she's finished school by LB, that makes her probably at least seventeen. If she has a winter birthday, she'd be a year older, but I think she seems young enough for a summer birthday.
Edmund is about a year older than Lucy, although that could be as many as two school years. If the "horrid school" where he begins to go wrong is the same boarding school Peter attends, his first year is pre-LWW and he is probably going on thirteen during LWW. Otherwise, his first year of boarding school is post-LWW and his second is PC, so he's probably eleven/twelve for LWW and twelve/thirteen for PC. Add a year for VDT/SC, and remember he's probably eighteen at least for LB.
Peter is studying for exams in VDT, but I've no idea if those are meant to be university exams or secondary school exams or what. Wikipedia (which as we all know is always accurate!) tells me that if he's sixteen he's going for his School Certificate and if he's eighteen it's his Higher School Certificate. Let's say fifteen/sixteen or seventeen/eighteen in VDT/SC, since he's still studying for them instead of taking them; that puts him at either fourteen or sixteen in PC and thirteen or fifteen in LWW. I think thirteen is much too young, so he's got to be fifteen in LWW and studying for his HSC in VDT.
