A planet’s age is a little less than Sun’s age of 4.6 billion years. Gas planets might be little younger than rocky planets.
Sun’s age is about 4.6 billion years.
Planets formed from planetesimals (minute space bodies) that come
together (coalesce) to form larger planets.
This could have happened at about the same period of millions of
years or later..
Earth’s age is about 4.53 billion years or a little more.
Gas planets’ formation from such planetesimals at longer
distances from vast solar surroundings could have taken a longer
period than that for the nearer terrestrial planets.
Reference:solar-center.stanford.edu/FAQ/Qage.html