In myth, as in fiction, there is no timeline. There are only timelines that later interpreters try to impose on the material -- violently, usually.

Ancient interpreters sometimes noticed this kind of anomaly too, and would maybe try to rationalise it away by inventing missing decades, that kind of thing. But it's still a secondary interpretation of the source material. Interpretations like that may have held some prestige in some quarters or may even have become "canonical", but that isn't the same as having it baked into the earlier source material.

Let's take a couple of extreme examples.

(1) According to traditional chronology, when Odysseus goes to war he leaves behind an infant son, Telemachos. The war takes ten yours, then he takes ten years getting home. Meanwhile, his son Telemachos grows up. Just at the point where he ceases being nepios (too childish to speak or be competent), and becomes ready to enter society as a man and speak to respected adults, suddenly that's the point that Odysseus comes home. Spot the problem.

(2) The case you bring up, of Achilleus being old enough to fight in the war, is even worse than you think. In the first place, I don't know what ancient backing you may have found for the age of Hermione, but it's not going to be any more solid than Telemachos' age. The Catalogue of Women has the marriage of Helen and Menelaos run straight into divine plans for the Trojan War without a break. The Kypria has the judgement of Paris take place immediately after the wedding of Peleus and Thetis, Achilleus' parents, apparently before Achilleus is even born -- yet Paris is apparently an adult. There's no timeline to be constructed out of this kind of stuff.

In the second place, Achilleus has time to engender a son before the war begins, Neoptolemos, and that son is old enough to fight in the last year of the war and be one of the most effectve warriors for the Greeks.

If you want to construct a timeline, don't make the mistake of thinking that it's baked into the story. Maybe it's the incoherence that's baked into the story, or maybe it's a result of different inconsistent stories being stuck together recklessly, who knows. Now, of course, that doesn't stop modern (and some ancient) interpreters from trying to rationalise the inconsistencies away. But let me paraphrase Eratosthenes and say that you'll find the correct timeline of the Trojan War when you find the cobbler who sewed the bag of Aiolos' winds.

When ancient interpreters do try to iron out these holes, their interest is basically scholastic or antiquarian. They're the kind of people who want to write treatises to work out where Hamlet was when his father was murdered.

When more "mythological" works notice the problem, by which I mean stories told in traditional storytelling modes, they typically adopt a very different approach. The basic idea is: use mythological tools to solve a mythological problem.

So a common solution is to have children grow up miraculously quickly. Now, we don't actually have that in any Greek mythological sources that I can think of offhand. We do get close. When Odysseus engenders his and Kirke's son, Telegonos, who ends up killing him without recognising him, that plays on a very ancient folkloric pattern. Other instances of this pattern, in the folklore and mythology of other parts of the IE world, regularly have the son grow up miraculously quickly, so that he's playing with 12-year-olds while still a toddler, that kind of thing - that's what we find in some Irish and Indian parallels.

By the time our extant Greek mythological sources were written, the authors already seem to have found this miraculous solution unacceptable. They've already removed it from the underlying pattern. So, as a result, they have incoherent timelines. That's part of the nature of the material. It's very different from problems historians wrestle with in trying to recover a historical reality.