It has aged very well. It's difficult for me to be objective, because I enjoy old shows and don't care if they look corny by latest standards, it's all about ideas for me. However, my girlfriend has never seen Star Trek until 2017, I introduced her to The Cage (TOS) and it was too dated for her (she's 27) even though she liked the premise. But after I showed her several TNG episodes, she actually got really into it, to be point of asking if we can watch some Star Trek - great win for me. She asked how old the show was and it surprised her, she thought it was more recent. But I'm being very selective in what episodes I show.
If you like Star Trek for the Sci Fi ideas, then I don't think that it really matters how it looks, that's just a bonus. I love TOS and watch it all the time, even though I was born over two decades after it was cancelled.
The worst one to age, imho, is Voyager, just because how relatively recent it is, yet it looks like it was made in the 80s. What's with the bulky laptops? It's supposed to be the 24th century. I can see the makers of TOS not consulting the few futurists that existed back then, but Voyager's computers looked older than the ones that were actually in use at the same time in the 20th century.
For those who watch ST for action scenes... I think those suck in all the reincarnations, to be honest. OK, the explosions are pretty cool in the reboot movies, but it's hardly the reason to enjoy STAR TREK.
Standards, standards, one-night shtandards. Anything with polish is going to look dated in 5 years, and the more polish is used to hide a bad story only makes the story look or sound even worse as a result. The actual stories and how well the actors translate what's on paper to screen is what truly counts.
I always found "The Cage" to be proto-TNG. It's slower paced in spots but is definitely more intellectual and Shakespeare-in-Space than a lot of Kirk-era TOS. A shame "Cage" didn't get picked up, but history might be a lot different as a result.
I always watch sci-fi for ideas, though by the mid-80s one could see how stagnant it would become, with TNG starting out with some of that stagnant feel but then making a quantum leap in terms of style and reviving the whole genre. What's funny is, back then I belittled it all as "soap opera". Little did I know that, almost 30 years later, TNG's level of soap is nothing compared to today's maudlin escapades that are closer to pure fantasy than anything even remotely sci-fi.
I like the semi-bulk of Voyager's laptops. If nothing else, in the mid-2010s, a lot of people are complaining about today's gear being so anorexic, with poor battery life, easy to break, overheating, and lack independent processing power. They're design issues trying to end run real life physics and engineering issues for the sake of "thin and shiny" or "form over function", which has its place but isn't a one-size-fits-all option. (Don't even ask about how great LED bulbs aren't once you factor in the lack of proper cooling heatsinks, you're likely not going to get 22 years from that bulb, restricted airflow renders any metal useless after a certain point and that bulb's base doesn't allow for much metal on top of everything else, but that's another topic...) Also add in Moore's law and other realities in "tech", it is reasonable to assume that things can only be shrunk so far before realizing real life problems and then making them bigger.
The reboot movies are a lot like "Nemesis" - rehashes, which are better or worse depending on individual aspects. What's even funnier is if you see "The Brady Bunch Movie" from 1995, you'll see the 2009-present movies use the same plot style (regurgitate elements from old tv episodes and you have a shiny new plot, sorta). Then again, DS9 and VOY both rehash TOS and TNG episodes at times as much as TNG rehashed its own on occasion. The only issue is of degree, right down to copy/pasting whole lines of dialogue to hasten the passage of time in the writers' room...?
I think if you separate the majority of the first two seasons of TNG, it has aged incredibly well. The first two seasons have aged poorly not because of low production value, but rather because they are just poorly written and executed. There are some exceptions, of course - "The Neutral Zone" and "Measure Of A Man" being two of the best episodes of Star Trek ever produced.
The later seasons of TNG was pretty revolutionary at the time, though. Especially once you get the sense that the cast really started to gel after some initial rocky relationships, TNG is marvelous. The addition of Guinan gave the series some staying power, and even Wesley evolved into a genuinely interesting character. I think TNG in general is second perhaps only to M*A*S*H in terms of TV shows that have aged wonderfully.
Blame the 90s, not Voyager. I find the 1990s aesthetic to be so ugly and unnecessary. Voyager's interior design is great, but every time I saw someone in a non-Starfleet uniform I just scream "OH MY GOD I HATE THE 90s!!!" In fact... I think it might just be the fashion angle... The shoulder pads, the weird patterns on clothing. DS9 and VOY both had these issues. TNG kind of got around it by starting in the 1980s, and also by sticking to a rather minimalist aesthetic with their interior design and costuming choices. DS9 and VOY were both like "How busy can we make everything look? Add random textures and patterns to every surface of this scene!"
TNG's mid-era (seasons 2-4) have aged the best. 1 is clunky, yet many of its episodes hint at more net
potential than 5-7 had, but lost out due to lack of rewrites. imperfect direction, or other issues attributed to that first year. 5 - 7 feel way too forced or are retreads of previously made classics ("Quality of Life" being the fourth or fifth variant of "The Measure of a Man" and is easily the worst redo of that season 2 classic.) Remaking season 1's clunkers might have been a better route to take, IMHO.
On DS9 I disagree; the clothing is so rich with design and color... it must have been a wardrobe designer's dream to get to be so creative. Ditto for set designers, DS9 is so lovely to look at both inside and out.
VOY went back to the monocolor boredom, except they chose dark gray over neutral mid-tone beige. As a result, VOY felt more like a warship as a result of a enormous hotel in outer space.
And yet, for all the complaints of the 1980s and beige, notice how anything to do with home sales are stressing monotone beige (late-80s) or sterile white/steel (80s, 90s) colors. Yuck. Even puke green and vampire red are more lively, and not in bad ways by comparison.
That's the problem with post-TOS Star Trek, to show distant future they take modern level of development and extrapolate it out a few years, so in 1987 they're showing 1992 level of technology. How does that make any sense? Why not reference any number of futurist projections for at least 2050 if they want to remain conservative.
TOS was in an era prior to the microchip's ubiquity. Once the microchip became mass-produced, think 1970s, advances and innovations of pretty much anything and thanks to the microchip (which innovates on the old vacuum tube technology) started to come much more rapidly than anything TOS dreamed of and it's snowballed since. Right down to using millions of microchips, each one operating more than 30 times per second, to run a display terminal instead of a cathode ray tube. Remember, "gurus" like Steve Jobs didn't invent anything. The makers of Star Trek had, forgot to patent (or decided that the ideas were so common that there was nothing to really make such staunch ownership of)... oh, and since nobody wanted to cart around a big box full of heavy and hot vacuum tubes (as a processor or as display mechanism via a CRT)...
That's the only real "problem", sci-fi writers are not able to think 50 years ahead as fast as they're supposed to compared to back then. And that makes absolutely no sense except it makes perfect sense. And shows yet another example of technology evolving faster than humankind is?