The trouble is, it's no good Appealing To Authority (pick a genetics expert, any one), as these "genetics experts" just don't agree with each other. Especially about the age of DNA samples.
There are two errors here. In the first place -- as far as I know, please disabuse if you can -- genetic experts all seem to agree that DNA
has an age even though it hasn't. There is a good reason for this. The fact that Out-of-Africa and other palaeo-anthropological theories are using DNA-aging is a huge boost for population geneticists in general. It is hard to bite the hand that feeds.
In the second place, again as far as I am aware, geneticists do
not argue about the age of DNA samples. What they do (apart from counting the mutations) is take historians' or pre-historians' word for the age of the
bodies from which the DNA is taken. This is OK (though pointless) if the bodies are tested using other means but since this is highly expensive most population geneticists use rough-and-ready paradigm assumptions. Thus DNA taken from an 'Anglo-Saxon' grave is assumed to be 1000 - 1500 years old. And compared to a 'Celtic' body which is assumed to be older etc etc.
What geneticists do argue about is the meaning of the variations they come across. The current state of genetic mapping is just about sufficient to distinguish one population from another but when it comes to saying which arrived first the geneticists argue bitterly (and no wonder since it is a mare's nest). Orthodox pre-historians of course use these internal disagreements as justification to ignore the whole science of genetics. Though it will sweep them away in time.