There are many methods to try to determine how an ancient language was spoken, all of them by definition hypothetical, but since linguistic change does follow sensical rules, and happens in gradual steps (i.e. you can almost be sure that the vowel /u/ won't shift to the vowel /a/ in a single generation, and I bet there isn't a single recorded case of that happening, simply because /u/ and /a/ are very far away from each other in the mouth and acoustically).

For instance, we have very solid evidence (but still circumstantial, and therefore this is technically an hypothesis) that the Greek letter η, that now stands for /i/, had a long eh-like sound, likely /ε:/, for this and due to many other changes, we can say that were a Modern Greek speaker to speak with a Greek from the 5th century BC, they would have understood barely anything, barring certain specific words. From tracing how the language changed, according to the extent evidence, we can probably say that this speaker, however, would have mostly (but with difficulty) understood a speaker from 12th century Constantinople in simple day-to-day conversations. At the very least, the two speakers would have the same sound inventories and similar grammar, but somewhat different vocabularies.

I still agree with you that saying one language is older than another is meaningless. At most we can say that a language is probably more conservative than others.