Informed speculation often IS science.

https://www.aau.edu/research-scholarship/featured-research-topics/scientists-confirm-age-universe-138-billion-years

'Scientists estimate the age of the universe to be about 13.8 billion years by measuring the rate of the universe's expansion and "rewinding" it to the Big Bang, using tools like the Cosmic Microwave Background (CMB) for the earliest snapshots and observing galaxy redshifts for current expansion rates. They also study the oldest stars and use models of cosmic evolution, such as the Lambda-CDM model, to confirm these estimates, with the CMB and galaxy data providing remarkable agreement.'