First you have to be able to identify the manufacturer and model. For some there are published serial number lists which will give the year of manufacturer. For others you may have to be satisfied to know a range of years when that model was produced.
No one number- sorry. In SOME cases, the age can be determined from a serial number- IF the gun has a serial number- but they were not required on rifles until 1968. In the case of Marlin rifles, it may be a single letter prefix to the serial number. In the case of Remington rifles, it may be a 2 or 3 letter code stamped on the left side of the barrel where it joins the frame.
7
18
You will have to call Mossberg
39
Winchester 1892 rifles were made from 1892-1945. To find the age, look on the metal plate, just behind the cock.
18 for rifles, 21 for handguns
The minimum legal age for any type of firearms ownership in Australia is 18.
18 for rifles and shotguns, 21 for handguns
Depending on the state they are in and the age of the child, hunting rifles.
In the US, 18 for rifles and shotguns, 21 for handguns.
1928