Eli5 if time is relative and gets effected by gravity how could we possibly determine the age of our universe?
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OP is under the assumption that because the universe is so big and so old that much of the available light that reaches us has been ‘tampered’ with by black holes along it’s journey.
The reality is that the universe is so much larger than you can possibly imagine. It is 99% empty space. It’s actually super rare that we see light that has been interacting with a black hole.
Most of the universe has experienced pretty much the same amount of time. The only places that have experienced meaningfully less time are regions really close to massive objects like black holes. We just measure the amount of time that has passed for most of the universe since that’s what we really care about.
Microwave background radiation and how far we can see tells us how much time the light has had to travel.
Over large distances of the universe, there is no gravity. The dents that gravity puts in spacetime average to zero, over large scales.
Therefore, the age of the universe is not impacted by gravity.
Actually time in the early universe ran slower:
https://www.scientificamerican.com/article/time-flowed-five-times-slower-shortly-after-the-big-bang/
What we measure is the expansion rate. Running back the clock until size zero gives you the age.
We determine the age of universe by measuring the travel of light. Right now with the James Webb infrared optical telescope we have seen as far as about 1 billion years after the Big Bang, lightyears of course. Interestingly the Big Bang theory was created by a Catholic priest, and it still stands.
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