When we age date a planet, we are actually just dating the age of the surface, not the whole planet.We can get absolute ages only if we have rocks from that surface.For example: a carbon-14 atom (the "parent") emits radiation and transforms to a nitrogen-14 atom (the "daughter").It is impossible to predict when a given atom will decay, but given a large number of similar atoms, the decay rate on average is predictable.When there is a scientific discussion about the age of, say a meteorite or the Earth, the media just talks about the large numbers and not about the dating technique (e.g. On the other hand, when the media talk about "more recent events," ages that are more comprehendible, such as when early Man built a fire or even how old a painting is (or some ancient parchment), then we bring up the dating technique in order to better validate the findings.
When the number of neutrons is not in balance with the protons then the atom of that particular element is said to be unstable.
We can then use radioactive age dating in order to date the ages of the surfaces (when the rocks first formed, i.e. We also have meteorites from asteroids and can date them, too.