How old is Neptune?
Short answer: About 4.6 billion years old.
Long answer: The Sun, the planets and almost everything else in our Solar System were all formed together from a spinning cloud of dust and gas. Our current best estimate is that this happened around 4.6 billion years ago, which means that Neptune is that old.
With a few exceptions such as moons and asteroids that may have formed more slowly, everything in our Solar System is the same age: 4.6 billion years old.
Author: Dave Owen
1. According to the "Nebular hypothesis". See en.wikipedia.org/wiki/Nebular_hypothesis