Yeah, he's too old, sure. But his attitude is very much that of a 19-year-old, and comes through and kinda makes up for it. It's also fun to still watch because this is where this (until recently) very long-lasting couple first hooked up, and it's fun to see the chemistry. (Although she and Kevin Costner had even more chemistry, so there ya go.)
Looking through IMDb's lists of popular celebrities from 1965 through 1969 (the movie was filmed in the fall of 1987, so someone born in 1970 would only have been 17 and a little too young), I think you just about nailed it with Affleck and McConaughey. They would have been able to play the goofball dumb jock very convincingly.
1965 - Robert Downey could have stolen this part. He was an active actor back then, and was quite manic with all the drugs he was doing. Yeah, he could have made it work. Maybe not quite athletic enough at the time though. I think he bulked up as he got older for Iron Man. Ben Stiller is similar, has the wild excitement, short, but was always pretty athletic. Charlie Sheen proved his baseball and psycho chops the very next year in Major League, so two big thumbs up for him. John C. Reilly could definitely play goofy, but he's nowhere near as good-looking as Robbins. Jeremy Piven was active at the time, but doesn't give off an athlete vibe. Chris Rock could play crazy, but he'd make it into a different movie.
Notably, Adam Sandler was born in 1966, and he too would have been excellent in the role. He is very athletic. Tim Robbins' buddy John Cusack is 1966 as well, and would have done well in the role - AND was actually an active actor at the time. I see Rainn Wilson and Diedrich Bader there as well, who also could have had some fun with the role.
In 67 you've got Will Ferrell, who would have made this into an entirely different movie, so, no, not him. Phillip Seymour Hoffman? Great actor, but "Let it rain!" would not have really worked here. Matt LeBlanc would have been good - didn't he do some stupid baseball movie? Harry Connick might have been able to play it, too.
68: Hugh Jackman? Definitely athletic enough, but he's an Aussie, and I don't think they really "get" baseball. Will Smith? Yeah, well, but it would definitely be a different movie, like with Ferrell, due to his constant wisecracking. Sam Rockwell - now there's a fun choice! Also a very different movie, though. Josh Brolin coulda pulled it off. Anthony-Michael Hall - wow! That would be weird! Sure, he was active at the time, but I would be constantly thinking about Sixteen Candles and The Breakfast Club. No way! Brendan Fraser could have done it, though. Owen Wilson - hmmm, no.
Finally, 69: Paul Rudd, Edward Norton, Jason Bateman - hell, no. Not goofy enough. Jack Black - waaaay too goofy, and doesn't quite fit the athletic look. Same for Zach Galifianakis. Matthew Perry mighta made it work. Javier Bardem would have changed the entire tone of the movie.
Hey, that was kind of fun! Thanks for "making" me do this!
I want the doctor to take your picture so I can look at you from inside as well.