^ I tend to ignore the age of the actors as strong evidence of any particular scenario, because it’s really just a very broad indicator at best, and doesn’t take into account any sci-fi/fantasy possibilities. But the reality is it’s tricky to extend the duration of the Empire too much over 19 years. You can say that Anakin/Vader turned evil before giving birth to the twins (and thus before turning into a cyborg). But presumably Mrs. Skywalker wouldn’t stick around once Anakin became a supervillain, so the only way to pull this off story-wise is to claim Anakin was “secretly” evil for some unspecified time period before the twins were born.
The “secretly evil Vader” scenario isn’t entirely implausible. Lucas himself has even implied this was the case in some old story discussions with Kasdan et. al., saying that the Jedi purge was sort of a stealthy affair, with Vader secretly catching Jedi off guard. (What did he like stab hundreds of thousands of people when they weren’t paying attention? I don’t know, maybe there weren’t that many Jedi.) I guess we can extend the duration of this “secret evil Vader” period to any number of decades, to extend the length of the Empire. But really, it stretches plausibility the longer we stretch it. Also, Lucas’ comments about “stealthy Vader” imply the Jedi purge happened during the waning days of the Old Republic, before the Empire.
I mean, believe me, I don’t like the idea that the Empire only lasted like 20 years. Before the Prequels, I kind of assumed the Empire was a very old institution, which is why taking it down seemed so impossible. I even came up with scenarios to explain how Vader could have turned evil much earlier than 19 years before A New Hope: maybe Ben Kenobi took Vader’s children and hid them by cryogenically freezing them for some unspecified time period. Then later (perhaps decades - or centuries? - later), when he thought it would be safe, he unthawed the twins and gave them to Owen/Beru and the Organas. But no… that doesn’t work, because Owen and Beru act like they were friendly with Anakin relatively recently.
I don’t know. It’s science fiction/fantasy. Maybe humans in Star Wars live for 500 years. Maybe Han Solo was actually 175 years old at the time of ANH. Who knows? Except, we all know that isn’t the intent here. It’s pretty clear the Empire is relatively young (despite certain impressions we may get to the contrary.) That’s the most straightforward interpretation of all the evidence.
Also, the deleted scene in ANH, where Luke talks with Biggs at Tosche station, actually gives the impression of a relatively young Empire. Luke and Biggs talk about how the Empire isn’t “out here” yet (i.e. not out on Tatooine), and is just now starting to “nationalize commerce”, indicating it’s still in a growth/expansion phase. Sure, it’s possible it takes centuries for Imperial control to expand to the Outer Rim, but that’s just not the impression I get from how Biggs talks about it - warning Luke that the Empire isn’t as far away as it seems.