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"The age of the universe is no more a theory than the length of your foot is a theory."

When you observe a car driving down the road at speed of 30 mph for and 1/8 of a mile, of coarse you know exactly how long the car has been driving for because:

You can know for sure exactly where that car started. It surely started in a town 30 miles away. That is of coarse not an assumption.

You certainly know for sure that the car has been traveling a constant speed for its entire life, nothing could change that. (and absolutely sure constant for 4 billion years)

So you know for sure that that the car trip, after observing the car for 1/8 of a mile has, has been traveling exactly 1 hour to 5 significant digits. Of coarse there are no assumptions in this. This is a measurement and "this is no more a theory than the length of your foot is a theory".

Of coarse you know with absolute certainty how much radioactive or daughter substance was present at the start, or where x started. Your invention of the very first time machine for going backwards in time to observe x is quite remarkable, the first of its kind, you should patent it.

Of coarse you know that the rate of decomposition, or the speed x, the always remained constant, and has certainly remained absolutely constant to 5 significant digits for exactly 4 billion years to 10 significant digits, this is
"this is no more a theory than the length of your foot is a theory".

Pardon the sarcasm, but you are making assumptions and calling it "no more a theory than the length of your foot is a theory".

Time is the escape hatch of your chance + enough time worldview, and given enough you time you believe that the absurdly improbable can certainly happen....just give it enough time. Be careful that you don't drown in your all- knowing assumptions, as your escape hatch may not be as open as you believe.

So you know for sure that that the car trip, after observing the car for 1/8 of a mile has, has been traveling exactly 1 hour to 5 significant digits. Of coarse there are no assumptions in this.

Of course there are assumptions. You ASSUME that the car's speedometer is functioning and properly calibrated. You ASSUME that the distance of the course is properly calibrated. You ASSUME that you know the diameter of the car's tires to five significant digits (bad assumption). But here's the key: every one of those needful measurements has an uncertainty associated with it. The time you calculate is no better than the most coarse of those uncertainties. That doesn't make it a theory, of course. You agree that it's a fact.

The measurement of the age of the universe proceeds the same way. It requires you to measure things like the absolute and relative brightness of Cepheid variable stars, the angular size of galaxies, the absolute and relative brightness of type 1a supernovae. These things have uncertainties associated with them, not to five significant digits, but--in decades past--to 10 to 50 percent. That means that the uncertainty in the final answer might have been no better than a factor of two. (Nowadays it's much better than that.) But still, that's only quantitatively different from the measurement you proposed, and which you called a fact. Qualitatively, they're the same.