system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know the universe is (only) 13.8 billion years old?
gpt
We calculated it by the inverse of the Hubble Constant (H)
v=Hd, where v is the speed of a galaxy moving away from us (Usually measured in kms^-1 and d is the distance to said galaxy (Usually measured in Mpc {Megaparsecs}). There's a lot of uncertainty in the measurements of these values, meaning a lot of uncertainty in H, values can range from 60kms^-1 Mpc^-1 to 80. 
Rearrange for H=v/d, convert speed to ms^-1 and distance to m, you'll see you have ms^-1 divided by m, so H is measured in s^-1. Inversing will then give you the age of the universe in seconds. Then convert to billion years and you have your answer for the age of the universe in a timescale we can sort of understand.
Fun fact: When Hubble was first calculating this constant (Which technically isn't a constant), his data was so erroneous that he ended up with an age of the universe younger than the age of the Earth - Luckily geologists had a rough idea of the age of the Earth at that point so Hubble knew his data was the problem.