I have read 13.8 billion. Either way how the fuck do they know?
13.8 billion is the closest they can estimate, it could be more or less and we’ll get closer to the true answer (although probably not with 100% precision) as more data/evidence is unearthed. But the 13.8 billion estimate is based on a couple of things, such as study of the cosmic microwave background (a radiation afterglow left behind a few hundred thousand years after the Big Bang), study of the oldest known stars we are currently aware of and by looking at the current expansion rate of the universe and by extrapolating back to the big bang