Sir Joshua Reynolds painting 'The Age of Innocence' is a charming portrait of a young girl. It is an extremely well-known image and has been reproduced many, many times. Some of these reproductions do have a modest value, however, the majority are very low ticket items. It sounds as though you either have an oiliograph print, or a hand produced copy. It's impossible to know for certain without seeing the item in person. It might, therefore be worth researching the value of similar items on Ebay, Etsy and liveauctioneers.com. You may well see something very similar on one or other of these sites.