How do we measure planet’s age?

In astronomy, the age of a star, planet, galaxy and its distance are linked: a young star very far away might not be visible to us, simply because its light hasn’t reached us.
Last week the Hubble telescope found the oldest, furthest galaxy, probably dating back to the Big Bang, which is quite exciting! All we can see is its red halo. The galaxy is not red, but as the Universe is forever expanding and the light is therefore bent as it tries to travel, it changes into red (will explain another time why!). By the hue of red we can tell how old/far is this galaxy. Cool eh?