How old is a German luger serial number 7914?

To correctly answer this, I think you need to know three pieces of data that is stamped on the gun:

1: Date of manufacture

2: 3-Letter Supplier Code. ex: "byf"

3: Serial Number -- as long as this is the serial number on the major part of the gun. Some guns are assembled from various parts of other guns and the serial numbers will not match. If they do match, then the last 2-digits will be stamped on several of the parts.