Sir Joshua Reynolds painted "Age of Innocence" in 1788. It hangs in the Tate Gallery in London. You may have a print, or you may have a painted copy. In times gone by, aspiring artists would make copies of famous works, and if you have one of these of a sufficiently high standard in a good quality frame, it may well fetch a reasonable price at auction. However, if you have one of the many, many prints in circulation, the value may well come down to the quality of the frame and is likely to be less than £100/$70. It is of course, impossible to tell without actually seeing the item in person.