What is the order of the planets in terms of age from youngest to oldest?

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