We can get absolute ages only if we have rocks from that surface.For others, all we are doing is getting a relative age, using things like the formation of craters and other features on a surface.These are the surfaces that we can get absolute ages for.For the others, one can only use relative age dating (such as counting craters) in order to estimate the age of the surface and the history of the surface.This may simply have to do with what the media is talking about.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 we age date a planet, we are actually just dating the age of the surface, not the whole planet.We have an activity in one of the PSI workshops "Exploring the Terrestrial Planets," that deals with this topic.So, you can use the radioactive elements to measure the age of rocks and minerals. Their useful range is from about 1/10 their half-life (the time it takes for half of the radioactive element/isotope-- the parent, to convert into a non-radioactive element/isotope-- the daughter) to 10 times their half-life. You can use this to measure the age of a rock from about 128 million years to more than 10 billion years (the Solar System is 4.56 billion years old).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.This predictable decay is called the half-life of the parent atom, the time it takes for one half of all of the parent atoms to transform into the daughter.