Jupiter is often thought of as the oldest, though the following classical explanation is complicated by recent exoplanet discoveries that don't always conform to the model.
In the classical model of the formation of the solar system, the protoplanetary disk from which the planets accumulated only had water as solid outside the "snow line", i.e. the distance from the Sun outside which water would exist as a solid, and inside as a gas. With some simple modeling you can deduce that the snow line would be at 2-5 AU shortly after the Sun's formation and that water ice would accumulate somewhere outside this radius.
Being an abundant volatile, the rapid accumulation of water ice allowed proto-Jupiter to be the first planetary body to collect the 7-10 Earth masses required to initiate run-away accretion of the hydrogen in the disk, forming the first and largest gas-giant in the solar system.
Stevenson D. J. & Lunine J. (1988) Rapid Formation of Jupiter by Diffusive Redistribution in the Solar Nebula, Icarus 75, 1, 146-155.Source