That is not always true, since it was the heat flow out of the earth that Lord Kelvin used to derive his estimate that the earth is tens of millions of year old.
But since you are clearly stuck on page one of this 500 page radiological dating idea, let me see if I can put it in terms that even a brand-new, just starting, recently graduated, hopeful-for-the-future, tinker toy engineer can understand.
A few days ago The Barbarian posted that Steve Austin, a prominent YEC scientist with good academic credentials, submitted a sample of material from the recent eruption of Mt. St. Helens to a laboratory for radiological dating. The lab that he selected not one that was equipped to date recently solidified magma, but Austin used them anyway. The sample he submitted to the lab was not a pure sample of just recently cooled magma. The date they came up with was millions of years, which is clearly nonsense for Mt. St. Helens. (post 5221)
In post 5245 you asked why a person would need to know the date of something before it can be dated.
(Now the analogies start) in answer to your question in post 5221, The Barbarian likened it to measuring temperatures of fudge or of a blast furnace. Barbie was not saying that the dating was a matter of measuring temperatures. He was saying that just like a low temperature substance (like fudge) needs one kind of thermometer, and a high temperature substance like molten steel needs a completely different kind of thermometer, likewise measuring recent dates in rocks required a different type of measurement than measuring dates of very old rocks. Get it? Low temperature fudge takes one kind of thermometer, and high temperature molten steel needs a different kind of thermometer, just like recently cooled magma has to be dated by one method, and very old magma has to be dated by a different method. That type of comparison is known as an analogy.
In post 5253 you asked how would you know the age before submitting it to determine the age?
Barbie, in post 5256 says you don’t always know the date, but Austin knew darn well that Mt St. Helens was recent, yet he submitted the sample to a lab that did dating of older rocks.
Kinda like if you are an engineer (know what that is?), and you specialize in designing dams that go across thousand-foot wide canyons. Then a customer asks you to measure something for them - the thickness of a strand of spider web.
You: “Whoa, my transit is designed to measure hundreds or thousands of feet.”
Cust: “It’ll be fine, just do the best you can with this spider web, and see what you get.”
You: “Ok, it’s your money. One foot (that’s as low as my transit will go).”
Cust: “Thanks, now I am going to tell everyone you idiots think spider web strands are one foot thick.”
(That was an analogy. I hope you see how using the wrong tool means you will get the wrong answer, whether it be in measuring spider web thickness or in dating recent lava or in measuring the temperature of hot liquids.)