There are really no monitors that are too old for external use. Serial terminal monitors that have a serial interface, that's about it. When you plug in an external monitor to a laptop running windows, remember to use the "win" + "P" key. This brings up the monitor source menu. You can select "computer," "duplicate", "extend" or "projector". Try projector, as this forces output to an external graphics port.
Also, If you are plugging in an HDMI monitor to DisplayPort and the HDMI monitor has a resolution above 1920x1080, you will need an active DisplayPort-to-HDMI adapter. Passive adapters will not be able to drive more than 1080p resolution. Also, make sure to set the monitor input to HDMI. Most recent monitors have multiple inputs. Make sure your monitor is set as HDMI as the input source.