So long as you have no prior history of mental defect, you should be able to purchase a machinegun as a citizen.
As far as age restriction goes, if you are old enough to be trusted with a 3,000 lbs vehicle carrying 20 gallons of gasoline and going at 80mph, you should also be trusted to own a firearm.
Regarding automatic weapons in general, current restrictions are ironically in favor of seasoned criminals who know how to shoot. If the common citizenry were allowed UZIs, which can be effective with minimal training and are cheap to own, my prediction is that criminals (and statists) would be far less bold in their activities. Nothing like a shower of lead to cool heels and improve manners.