First off, you didn't specify which army, nor did you specify the country of manufacture - the SKS was manufactured in the former Soviet Union, Romania, East Germany, North Korea, Vietnam, the Peoples' Republic of China, the former Yugoslavia, and Albania. A lot more information would be needed in order to determine this. Bear in mind that not all SKS rifles manufactured saw military service - many of the M59/66 rifles were never issued, and China continues to manufacture the SKS for the commercial civilian market.
Some SKS rifles were brought back... most of them were manufactured in East Germany or China, as the Vietnamese made SKS rifles were and are extremely rare. They were either captured or sold/traded for favours to other soldiers in-country.
They began production of the Type 56 carbine (the Chinese designation for the SKS) in 1956, and production (for the civil market) continues to the present day.
18, just like any other rifle.
Federal law is 18 to purchase a rifle. Ask your local police if Atlanta has a prohibition on the SKS.
Up to 10 years in prison. The only country still manufacturing the SKS is China, and they were prohibited from firearms imports in 93 under the Clinton administration, so a Chinese SKS in this country is at least 21 years old.
SKS Microfinance was created in 1998.
The D and M models accept AK47 magazines whereas the standard sks has an integral magazine.
The only SKS carbines currently which require registration are the Chinese-made SKS-M and SKS-D models. Those could not be imported into the state after 1989, and had to be registered by 2000.
All of the fixed ten round mags are cross compatible, but cannot be used in the SKS-M or SKS-D, both of which use a detachable AK magazine.
Very similar, minor differences,
Beretta 92F.
Value of ANY gun is based on exact make, model, and condition. A well worn Yugo SKS may be $150, while a pristine Russian SKS is $500. An average Chinese SKS is around $250-$275.