well, 21 for most companies for renting car. some prefer drivers over 25 or so. Or maybe I'm just confusing countries..
you could get US driving license as 17yo. Just that I don't think paperwork would be possible due to being there on visa waiver or M1 with short duration. US DL tests are primitive. All you need to know is some general knowledge, common sense and the meaning of 4-way stop :-P
Bikes are OK, but it ain't fun riding bike in summer. Maybe SoCal is better, I've only been to Georgia. Pretty sweaty after stopping (no fan effect of passing air). But for a month? Bike's just fine. If they don't have any spare bikes, buy one in some 'mart' and then just return it after month that you changed your mind. That's the beauty of US malls. It's broken? warranty.
It's OK? You don't like it. IT stuff got only 14 days return even in Wall-mart.
Underwear is not very returnable cos' of change of mind.
Bikes should be just fine. Or you can spare those 70 bucks anyway..
Some folks mentioned on other threads that AAA have nice cross country routes for students. Enjoy. I'm off to Oregon in June :-P not planks