They don't determine the age. Most accept the prevailing paradigm and interpret results accordingly. There are also astronomers and astrophysicists though who interpret the data and 'determine' that we live in a young universe.
Astronomers make the observations and aren't involved in age determinations.
Astrophysicists model the behaviour of objects, such as stars and galaxies, and they use nuclear physics to calculate the ages of stars which would predict properties that match astronomers' observations.
Cosmologists are looking at the behaviour of the universe as a whole, and their age determinations are made using observations of expansion rates, the spherical geometry of the universe and the finer power spectrum details of the cosmic background radiation.
I am sure that there are more cosmologists called Steve who determine billions of years than there are cosmologists or astrophysicists who believe that the data shows a young universe.
You are puffing with wishful thinking again. Look at the data: what is there in the astrophysics/cosmology world that does not require an old universe?