How can we be sure about the age of the universe?

If the Universe is expanding faster than the speed of light then that would mean certain objects exist now that will never be seen here on Earth
 
If scientists measure the age of the Universe by observing galaxies/objects that they can see then how can they be sure of the age of the Universe if there are (theoretically) many galaxies they can't and never will see?
 
If you see what I mean...
 
Thanks
 
Paul