I very reluctantly disagree. I'm in my mid-30s (born 1988), and I watched Alien for the first time about a year ago. I have to say on the whole, I didn't like it. I think it's a victim of being just on the cusp of being "too old" for modern tastes. Like much other media of its era & older, it's paced too slowly. Too many long panning cuts, too much staring at a wall, too much space between dialog, too little action. I ended up staring at my phone for the 2nd half of the movie, wishing it would be over already.
There are some other films of a similar timeframe that do hold up. I thought Jaws, released 4 years before Alien, was paced absolutely excellently, and I'd call it one of my favorite films. But Alien feels more like older media in its pacing, and it suffers for it today. I absolutely do not deny it is an excellent film "for its time." Obviously the special effects, acting, set design, and music are all wonderful and hold up on their own, and it's undeniably a very important film historically & culturally. But much like Citizen Kane or original Trek, I think it's just a bit too slow for a general audience in the 2020s. The sequels feel better today.