When and what was the reason, for the first restriction on the 2nd, to wit; .. a felon cannot own a firearm ... and simular restrictions.
Nothing has changed.
Many of these restrictions come from Title I of the Gun Control Act of 1968, which you can find under 18 U.S.C. - Section 922, which governs who can't own firearms as well as restrictions on their manufacture and sale. This law also determines the age of the buyer.
There are parts of this law that are overreaching, but we realistically do need some regulations on the purchase, transfer, manufacture, and ownership of arms. I'm not particularly crazy about the idea of a 14-year-old gang member purchasing a fully automatic Uzi or jihadis openly carrying RPGs on planes.
Someone here said that this not the job of the legislative branch, and I would partly disagree. The interpretation of the Second Amendment is clearly a function of the courts, but do we really want the least democratic branch determining all the boundaries? The courts really aren't capable of doing that, and it would be scary if they tried.
The original definition of ‘felon’ was someone convicted of a crime punishable by death or forfeiture of land (pretty much the same thing back then). The King was doing you a favor by letting you live, so you had no right to complain about how you were treated. The problem is that the original definition has been so watered down that practically anything can be a felony, but the loss of rights remains.