Do all bikes have frame numbers?

Almost all bicycles have a unique identification number engraved into their frame. This number is usually between seven and ten digits long, and its called the “bike serial number” or “bike frame number”.

How can you tell how old a Trek bike is?

If yours is of this kind near the bottom of the linked page you will find information on how to determine the year and manufacture date of the bike. The serial number for the bike will be found on the underside of the bottom bracket shell which it the tube of the frame that the cranks attach to.