Confirming what a couple people have said regarding the U.S. military as well as adding a few more things...
Assuming you can find a way to get residency in the U.S. (finding someone to marry is probably the easiest way
), you shouldn't have any issues joining the military. Only the clearance will be an issue until you gain citizenship. However, being in the military allows you to apply for citizenship under different rules, the most important of which is that instead of having to wait for 3 to 5 years (depending on your type of residency), you only have to serve in the U.S. military for 1 year in order to be eligible for citizenship. If you serve during times of hostility (such as right now), you don't even have to wait a year. You might find this interesting: http://usmilitary.about.com/od/theorderlyroom/a/citizenship.htm
As far as age, that's already going to be an issue with the Marines. For the Army, however, you still have a few more years. I think the limit is 35 right now. I don't know exactly what you're looking for (support, infantry, elite, etc.), but regardless of the age limit for the Army as a whole, the age limit for special forces is 30. You can, however, still go to Ranger school, air assault, jump school, etc., and there are some pretty hard core units out there. 10th Mountain out of Fort Drum, for example, is top notch in my opinion, and they'll probably even give you a chance to go to the Army Mountain Warfare School and, after that, the assault climber school, in Vermont.
To be honest, I'm actually asking myself many of the same questions. I'm 34 years old and have recently moved to Quebec to be with my girlfriend and son, but I'm still on drill status with the U.S. Army, having put in 6 years of active service and going on 6 years of National Guard service. Now I'm considering joining the Canadian Forces, but I'm having second thoughts. There is no doubt in my mind that I don't ever want to be anything other than a soldier. Civilian life bores the hell out of me, and I love the looks of the CF overall. But starting over at age 35 means I'd be an old man by the time I could get a decent pension out of it, or I could convince my girlfriend to let me go back to the U.S. Army and finish out my retirement there. Either way, it's all about finances really. We're all on the same team, more or less, fighting the same fight. That said, other preferences aside, if you're looking for action the U.S. military has plenty of it.
Also, the pay difference... At first glance, I looked at the pay chart and got the impression that the Canadian Forces pays considerably better. But after learning how deductions work, that's adding to my second thoughts. In the U.S. military my base pay is slightly lower (at a similar rank), but if I have a family and live off base, I get an untaxed basic housing allowance of about 1k to 1.5k depending on where I'm stationed. In the CF I'd make a slightly higher base pay, but I'd just receive less deductions for living off base with a family.
But anyway, just sharing my thoughts with you since you may find yourself considering the same things
Ultimately, financial issues aside, I'd be proud to serve either country.