If you are talking about your thermostat like ^ said, it can take a few minutes to get warm. It is possible for your thermostat to stick open causing the warmup to take extra long. Finally it could just be your gauge or sending unit. Unplug your sending unit, located close to the thermostat housing and ground the wire with your key on, if the gauge is good, it will peg to hot.
2002 Black GT (Spectre)
2011 Red Candy Metallic V6 (Manfred)
2016 Magnetic Metallic GT (MkIV)