Well nice thinking ,You are right in thinking they were older. But let me explain.
Our solar system started as a large cloud of dust and gas called Nebula. Over many billions of years, the immense gravity of this large cloud caused the dust and gas to slowly fall inward towards its center. The center continued to collect more and more matter, growing larger and larger. At the same time, smaller clumps of matter began to form throughout the disk. These smaller clumps eventually became planets, moons, asteroids and comets.
As matter collects into clumps it heats up. The more matter that collects, the hotter an object becomes. The Earth is still very hot in its core, this heat is left over from when the Earth originally formed. Eventually the Sun became so hot in its core that it ignited, turning hydrogen into helium.
As the Solar System formed, the dust and gas in the inner Solar System was pushed out much sooner than the gas and dust in the outer Solar System. The outer planets formed in an area which was richer in dust and gas.
The planets closest to the Sun, (Mercury, Venus, Earth and Mars) are called the terrestrial planets. They are small, dense, rocky planets. The outer planets (Jupiter, Saturn, Uranus and Neptune) are called the Jovian Planets. They are large and made up of gas.