I haven't kept track of this very much, and am curious: why might the fork not have happened if Ethereum was older? Assuming the undesirable transaction happened relatively close the the point of forking, only a small number of descendent transactions would need to be reverted, right?

No transactions were reverted. The rules were changed so the account with the "stolen" ETH could be drained to an address without the owner's approval.

But for this to happen, a hard fork was needed and if enough of the miners would have rejected that, it wouldn't have happened.

In part, this is exactly what happened. The hard fork was done but a group of miners used code without the new rules, therefore creating a separate blockchain.

Ethereum is the Ethereum blockchain where the DAO hacker was drained, Ethereum Classic is the Ethereum blockchain were their account wasn't drained (lots of turmoil followed on the ETC chain but that's a different issue).