I think the whole point of IQ test is being stable independent of age.
You don't have an IQ. It's a test that you take that can provide some useful information, but it isn't a fixed score, and "intelligence" (a word that is too ambiguous to use without careful definition) can't be reduced to a single number.
If we don't call one's skill at mental tasks 'intelligence', just how should we label it?
It will take me a long time to write out my thoughts completely, but I plan to do it at some point.