I have several old Lego sets, new in box. Maybe 20 years old or more. How to determine the age and value of these old Lego sets?
I would suggest going to Bricklink and entering the set numbers in. You will find the information on the sets you have and, if it isn't extremely rare, a price guide with information on how much the set has sold for in the past and how much it's currently selling for.
There is a lot of information on this site that you may find useful, here is a link to one of the more recent inquiries with an answer and more information.
An alternative to using BrickLink for checking resale values is to use BrickPicker. BrickPicker bases its “current value” on Sold prices from EBay, which sees larger traffic than BrickLink. You do need to count on paying for shipping, and paying 13% of the revenue (including shipping costs) to EBay and Paypal. BrickLink charges significantly less, and shipping is paid for by the buyer.
If you have a large list of LEGO sets you are seeking to valuate, BrickPicker offers a bulk lot evaluation tool which saves you time.