|Look for reviews of the model in Google. That will usually reveal the age. The OS the laptop came with can also be used as a gauge. The chip set in use will also help but that information is as readily available.|
You had better be very careful buying used laptops. There are many stolen units out there.
You may not be aware of this but with all the security on modern laptops you may not be able to even boot or assess the hard drive. Additionally, if you don't receive restore disks you may need to BUY a copy of Windows to get it running.
With the price of new laptops I suggest you rethink this idea.
Almost daily we get posts asking for help with laptop passwords. These are not easily, if at all, dealt with. Hard drive passwords most likely require a drive replacement. Even then you may not have full function. All in all a nasty business.