Age of the universe

How do they calculate the age of the universe

They use Hubble constant (Ho) to estimate the age of the universe..

But How Do They Do It..?

This is how...
Ho = ratio of the velocity at which a galaxy a receding from us to the distance of the stellar object. Though the velocity at which a galaxy is receding from us might differ depending on the distance from our galaxy, but the Hubble constant should be the same irrespective of the galaxy under consideration. (notice, the unit of Hubble constant is in per time)

Say a galaxy is receding at the rate of some few Mpc per year (mega parsec per year) and the distance to that galaxy from milky way is estimated* to be few million parsec, then this ratio of velocity to distance will give us the Hubble constant with a certain confidence interval. I say a certain confidence interval because of the inaccuracies involved in the methods employed to estimate the velocity and the distance.

Now, the reciprocal of the Hubble constant will give the estimate of the age of the universe (with the same confidence interval). So better the estimate of the Hubble constant better the guessitmate* of the age of universe.