All radiometric dating is based on several assumptions that are very questionable.  For radiometric dating to work, all of the following must be true.
1.  The exact proportion of parent and daughter isotopes have to be known (how can we know when we don't even know the date?)
2.  We have to know the exact decay rate and that decay rate has to be constant even under changing conditions (this is the easiest to believe, but there is still physical evidence that it isn't true).
3.  The sample must not allow any of the parent or daughter isotope to come into or exit the sample (since most are water soluable, this is hard to believe).

C-14 dating uses the assumption that we currently have a particular proportionality of C-14 and C-12.  This proportion is present in the CO2 in the air that plants use and therefore the plants have this same proportion in their flesh.  Animals eat these plants and therefore have the same proportion.  When either dies, they stop ingesting C-14 and therefore it starts decaying into C-12 over time.
There are two problems with this dating.  Plants and animals in the ocean are further from the initial ingestion of C-14 and therefore they (and anyone/anything that eats them) with have a lower proportion of C-14 giving an older age when their age is calculated.  Also, Earth's magnetic field halves approximately every 1400 years.  The magnetic field was higher in the past.  This reduced the solar radiation which converts N-14 to C-14, therefore the C14/C12 ratio was lower in the past, once again giving older dates.  There is no measurable C-14 after less than 100,000 years and isn't very accurate over about 10,000 years.

Since rocks aren't ingesting carbon, these symptoms clearly don't work, so other radiometric methods are used, most of which have extremely long half-lives meaning there is not much change over millions of years.  They also tend to assume the sample started with only the parent isotope (which is very unlikely).  Most (if not all) cases that radiometric dating was used to test rocks of unknown ages, it gave multiple orders of magnitude higher ages.  If it doesn't work on rocks of known ages, why should anyone believe the ages of rocks of unknown ages?