See, the thing is a lot of this is just the stimulation of first experience, and LESS experience overall. It's the same reason why summers as a kid used feel long and awesome--and now they are short and barely memorable. Because the proportion of your experiences was extremely limited when you were younger. As you grow older, you see patterns repeat, you experience things again and again, it takes more to make an impression.
One of those movies you watched back then may have been judged against maybe, depending on your age, about 100 films. Today, it will be one of a thousand, maybe much more depending on how big a movie buff you are. You've seen the patterns, you've seen the emotions, the characters, anything that's the same is less stimulating. How many times have you gone back and watched a movie that as a kid you thought was the most awesomest thing ever? And then as an adult thought "holy fuck this is bad". Why? Experience. And experience works the same way on new movies, too.
Tell me a 15 year old Astro doesn't walk into Doctor Strange, without ever seeing any other super hero movie, or not many movies at all, and isn't blown the fuck away. You, as an older person, have seen Dr Strange before, and done better--so you judge it more harshly, and this is the cycle that makes old people seem bitter. It's just experience.
It works in reverse too though. How many people told me to watch Citizen Kaine because it was amazing! But its not...not really. It's good, story is good, the movie is well put together, but its nothing exceptional...to me. But when that movie came out, a lot of the techniques in it were brand new, and amazing. It made a massive impact on the people who saw it, and that "new" experience, left it's mark. The same with Star Wars, a kid today watches Star Wars and "it's a good movie"....because it is...in that timeless, classic way, it has all the hallmarks of a good movie. But are they as blown the fuck away by it as we were? No. The special effects that completely changed our world because of how new they were are old hat to kids today, Star Wars is a classic that uses techniques which have now become normal.
That's the way life works. It's why happiness is fleeting, it's just the thing before you need more.