I don't know. This is a hard question. I'd like to think they'd be on the same side, though I can imagine some scenarios that might bring them into conflict, probably over some priceless artifact they're both after for different reasons.
As for the age difference, well, the actor who plays Rick O'Connell is younger than Harrison Ford, but I'm not sure there's a big difference in the characters' ages. I know Indy was supposed to be about 36 in 1936. I'm not sure how old O'Connell was supposed to be when he first appeared (in the battle at Hamanuptra (sp.?) in 1923. Older than 23, I'd say. Probably late-twenties. He might actually be older than Indy. He does appear to be a little bigger, but not that much bigger.
They're both very good gunmen, though O'Connell's two-gun style struck me as a little fanciful, while Indy's technique was more believable, especially for the period. (Lots of people carried two guns, but very few ever fired both simultaneously.)
Well, a few minute's Googling around revealed that both characters were born in 1899. So they're basically the same age, give or take a few months.
Mervyn