I've seen a couple over the years: Robin Hood, Twelve Chairs, Blazing Saddles, and most recently Spaceballs. I found them all funny, theoretically, but they rarely got an open laugh out of me. It seems like most of the time this is just because the comedic timing makes every joke or funny bit land very awkwardly, or with just a bit too much space to leave room for any subtlety or reward for the watcher.
I feel like I remember the few I saw as a kid being funnier at that time. Is it just because my brain has been attention-poisoned with the rapid fire wit of modern comedic television?
A while back, I took my niece to the drive in movies for her birthday. She loves star wars, and it was the empire strikes back and spaceballs. I thought she'd get sleepy and the humor in spaceballs would go over her head, so I thought we'd be leaving early. Nope! She absolutely loved spaceballs and was howling with laughter. I laughed too, but mostly because she was making me laugh. I don't know the answer to your question, but I can tell you that at least one child still loves Mel Brooks. So maybe it is the brain poisoning? Dunno.