Keaton's first feature (as a writer/director, at least) doesn't quite live up to the promise of his shorts, probably because he hedges his bets when it comes to really spoofing Intolerance. That is, what makes Intolerance such an amazing experience is the way that as the movie goes on not only do each of the three major threads pick up pace, but the inter-cutting between them becomes more and more frequent, until all the stories are climaxing at the same time (fitting for a movie with one of the most famous orgy scenes in film history). But Three Ages never really comes together like that: you get a chunk of the Stone Age story, then a chunk of the Roman story, and then a chunk of the Modern Story, repeat. And it doesn't help that each of these "chunks" is essentially the same: that's part of the joke (Buster faces the same problem in every Age) and it makes sense thematically ("Love" is a constant throughout history), but, dramatically, it's a little dull. I can't help but imagine what it could have been: cutting between three slapstick set pieces, so that, through editing, they all come together in some way.
It's also a bit of a problem that the three individual stories aren't great, either. They're not bad by any means, but Keaton's other shorts were mostly better than them. Part of my problem may result from how Three Ages is packaged on the Kino DVD: it's followed by The Goat, which may be one of the greatest short movies ever made. The Goat is so inventive that it makes Three Ages seem a lot less impressive. The gags in The Goat are brilliant and (often) laugh-out-loud funny, but the gags in Three Ages are clever and amusing.
Still, it's nice to see a movie comedy that's made on a relatively large scale (the Roman sets could have been used (were used?) for a genuine historical epic) that actually deserves that scale and knows how to make use of it. It's a nice change from today's big comedies, like Talladega Nights, where the huge Nascar set-pieces are handled with the minimum of wit and invention and the hugeness production (not to mention the budget) undermines the goofier, no-big-deal charms of its stars.
I think there are quite a few really good comic actors working in movies right now - Will Ferrell and John C. Reilly and Sacha Baron Cohen among them - but most of them seem more suited for "tossed-off" productions, like the Martin & Lewis movies or 1930s Paramount comedies, or, I guess, like Borat. But there doesn't seem to be anyone in Hollywood anymore who can make a truly "through-composed" comedy (especially since the Farrelly Brothers seem to have lost their way).