In short - No.
In most articles you read everyone says that a plus point for cave living is the constant temperature. Depending on the article this constant temperature varies between 18 and 23 degrees all year round suggesting that along with a cave structure being unique, to a certain extent so is the caves ability to maintain warmth.
While I am not trying to blow this theory out the water altogether it may worth considering for first time buyers that caves do get cold. In fact it is the outside temperature that is more important. To give an example this year temperatures dipped to around minus 12 in February and rose to 45 in late July. With extreme heat outside 20 ish degrees will feel quite cool when you walk in the front door and the reverse from sub zero to a comfortable 20 degrees inside.
The caves temperature is not constant - it varies although nothing near like a conventional house. In my own cave the temperature is currently 21 degrees (4 degrees lower than a month ago) and by the winter it will have dropped to around 17. One other thing is that if you have a long cave that goes deep into the earth instead of a broad cave mostly nearer the front of the rock the temp will also vary by around 2 degrees from the back to the front - the front being the warmest point.
Again, each cave is different and so will be its insulating qualities depending on how much rock you have above you and to either side, alot will also depend on thickness of walls particularly on the facade. This year the fire will probably get lit in early December, it will be comfortable until then. So to sum up it all depends on the cave but dont believe 18 degrees all year, it may fluctuate from anything as low as 16 to as high as 26 depending on the weather outside.