The poor July baby strip specifically establishes the three main girls’ ages as of the end of “The Case of the Lonely One”, and pins down their birthdays to at least within a month (more precisely for Mildred and Shauna).

At the end of “Lonely One”, which is explicitly dated to September–October of the kids’ second year, Mildred is 13, having turned 13 on September 1st, a few days before the start of the case. Shauna is 12, and won’t turn 13 until the next day (which I think, from specific dates given earlier, is October 11th, but there are a couple of assumptions involved in figuring that, so I might be off by a few days). Lottie is still 12, and won’t turn 13 until July. It’s specifically noted that she’s nine months younger than Shauna, and also established that she’s ten, or maybe even eleven, months younger than Mildred (we never learn exactly when In July Lottie’s birthday is, though I’m guessing it’s not the 18th, or she would have commented on it in “Forked Road”).

Shauna’s 12th birthday is specifically noted during “The Case of the Team Spirit”. (That’s also the strip where the legendary purple puffer makes its first appearance.) The nice blue coat she gets for her birthday becomes a plot device in that case. Lottie is nine months younger than Shauna. When Shauna turns 12, Lottie is 11, and will be 11 until the next July, which is sometime round about “The Case of the Simple Soul” (the day the girls take the troll to the pub is the last day of school).

From there it’s just a matter of counting years. Unlike Bobbins and Scary Go Round, which operated in kind of vague webcomics time, the passage of time in Bad Machinery is well-defined, marked out by school terms. Lottie is 12 in “Lonely One”, which is the autumn of their second year.

Lottie is still 12 in “Fire Inside”, which starts on New Year’s Eve, though Mildred is again noted to be 13, and Sonny is also established to now be 13, with his birthday on Christmas Day.

“Unwelcome Visitor” is set during the summer between their second and third years. Lottie has finally turned 13.

“Forked Road” is in the autumn of their third year. The first trip through the time hole was October 30th. Mildred and Shauna are 14, but Lottie is still 13.

“Modern Men” is in the spring of their third year. Sonny has turned 14, but Lottie is still 13.

The summer between third and fourth years is the story-within-the-story of “Big Hiatus”, assuming that it happened at all. But even if it didn’t, Lottie turned 14 around the time that it’s set.

The autumn of fourth year is the Bobbins story “Into the Woods”, which is mostly set in November, with the epilogue, including The Boy’s second death and Mordawwa showing up on Scientist to tell Shelley that she’s pregnant, in February of the kids’ fourth year. Mildred and Shauna are 15, and Sonny has turned 15 by the epilogue, but Lottie is still 14. And note this puts the rest of the series on a hard (yards) timer: Peggy is born no later than November of the kids’ fifth year, and Bad Machinery gets time-fixed to this by the “Big Hiatus” framing story (which was the day before “Severed Alliance”, and immediately after a specific “Hard Yards” strip; Lottie just missed Erin leaving Shelley’s) and the kids’ involvement in “Hard Yards”.

“Space is the Place” picks up in the spring of the kids’ fourth year. Shauna is 15, but it isn’t July yet — because there needs to be room to fit most of “Missing Piece”, including a two-week timeskip, in before end of term — so Lottie is still 14. As I noted above, Shauna identifies them as “two 15-year olds” after the simulator sequence, but while Shauna is 15, Lottie isn’t quite yet.

“Missing Piece” overlaps “Space is the Place”. Blossom’s coup happens while Shauna and Lottie are away in Wales. The interstitial strips that establish this were cut from the published version, but nothing was added that contradicts them, so I’m assuming it’s still the case. Shauna and Mildred are 15. Lottie is still 14 at the beginning of the case, but turns 15 sometime around the end of it, which is the last day of their fourth year.

“Severed Alliance” picks up a few days later, in the summer between fourth and fifth year, around the beginning of August. Lottie has recently turned 15. Mildred is still 15; Lottie says in the second strip that Mildred won’t be 17 (when she can get her driver’s license) for a year and a month. The very last strip is the first day of their fifth year; Mildred has just turned 16, but Shauna and Lottie are still 15. Shauna turns 16 a month and a half later. Lottie won’t turn 16 until the next summer, around the time of the Case of the Jade Cartel. So the events we’re currently witnessing are the winter Lottie was 16, which was the year after Mildred turned 16, because Lottie is at least ten, maybe almost eleven, months younger than Mildred.

The kids aren’t always 100% precise about stating their ages. In addition to the one I noted above where Shauna lumps 14-year-old Lottie in with herself as “15-year-olds”, the most obvious one is in “Lonely One”, where Shauna tells Lem’s dad that she’s 13, but the “poor July baby” strip establishes only a few strips later that she’s actually still 12, and was jumping the gun by a few days.

This particularly applies to ages when things happened a decade earlier.