I love these old school desks!! They are one of those things, like pianos, that don't seem to have a desirable function in modern homes though, and the prices on them are not what you would expect. I follow them quite a bit on our local craigslist, but like most people, I just don't have a good place to put one.
Usually I see people start out listing a desk for $100 or even $200 and it doesn't sell. They are too large to ship reasonably on Ebay, so they miss out on that market. Then the price drops and typically stops around $50 to $75 in the San Francisco Bay Area. However, just last week I saw a beautiful pair, still attached to their original floorboards from the old schoolhouse selling for $15.
Sadly, I think these wonderful pieces of history are terribly undervalued. I love them and love to think of all the little kids sitting in them, wishing the lesson would end!
I will let the experts respond on the age. I think yours may have been refinished, these are usually pretty battered after years of use by kids. But I could be wrong on that. Can you show a clearer photo of the desktop surface? That should be a good indicator. These are one of those items that refinishing doesn't seem to affect the value much either way.