the younger the better! Probably around 2-3 months? You'd probably want them to have at least their first set of vaccinations (if you are getting them done)...

Not the same thing, but we went to Melbourne (a 6 hour drive) when DD was only a couple of months old and she slept the whole way, except for waking up twice to have a feed. These days she is lucky to travel an hour before she wakes up and gets cranky of being stuck in her car seat. Will be easier once we can turn her around but I dont really want to do that just yet.