What is the age and value of a Belgian 16 gauge Sweet Sixteen serial 1845 with an A above it?

It was made between November 57 and April 1958. Records are sketchy with the "A" prefix Auto5's, and this is the closest that we can get. FN made over 10,000 in this A & T series during the period. Being a lower number, it would be safe to assume that your gun was made in 1957. Value depends on condition. On the low end a well used Sweet would fetch around $400-$600. A mint in the box could bring $1800+.