Could an Earth-sized planet be cold? This question poses the question of why the Earth is still hot after 4.5 billion years, and the primary reasons were:


  (1) heat from when the planet formed and accreted, which has not yet
  been lost; (2) frictional heating, caused by denser core material
  sinking to the center of the planet; and (3) heat from the decay of
  radioactive elements.


With that in mind, would it be possible for a planet of Earth's size and age to have a cold core, mantle, and crust?
If you peruse Google for Earth cooling rates, you might find what you're looking for. Here's one of many sources.  I strongly recommend reading the entire page,as it discusses some of the geology-based estimates as well.


  Kelvin's primary attack on geologic dating was that measurements of
  the rates of geologic processes were highly uncertain, if it was
  possible to measure them at all (Hallam, 110). Accordingly, Kelvin
  sought to apply the knowledge of physics to the problem. Given that
  temperature increases the further one descends below Earth's surface,
  Kelvin concluded that Earth was slowly cooling. He set out to
  calculate the time required for the Earth to cool, and thereby
  solidify, from an initially molten state. The idea that Earth had
  begun as an incredibly hot sphere of liquid dates back to Descartes
  and Leibnitz. This assumed initial condition was the linchpin for
  Kelvin's entire method (Hallam, 110). Bits of material at the surface
  would sink before solidifying, creating convection currents that kept
  the Earth at a uniform temperature until solidification began at the
  core (Hallam, 110; Knopf, 445). Kelvin needed to know: (1) the
  temperature at Earth's core, (2) the temperature gradient with regard
  to depth below the surface, and (3) the thermal conductivity of rocks.
  The gradient was established to be around one degree Fahrenheit for
  every fifty feet. Kelvin made his own measurements of conductivity.
  The problem was determining the temperature at the core. This is where
  Kelvin's theory of solidification enters the picture. Because the core
  was thought to be solid rock, its temperature could not exceed the
  melting point of rocks (Hallam, 110). He constructed the following
  equation:
  
  dθ / dx = S / h √(π t)
  
  The temperature gradient is expressed by dθ / dx; h is the thermal
  conductivity; x is the distance below the surface, and θ is the
  temperature (Holmes, 445). In 1862, Kelvin arrived at a likely age of
  100 million years. Because of uncertainties in the data, the lower and
  upper limits were 20 million and 400 million years (Dalyrmple, 26;
  Hallam, 111).


That said, there are plenty of more recent papers which estimate the cooling rates (with and without the help of the radioactive elements) based on the kinds of data now available as to the current and historical makeup of the core, mantle, and outer layer(s), heat transfer cycles, etc.