How old is Venus? Scientists actually think that everything in the Solar System was formed at the same time, about 4.6 billion years ago.
Before that point, our entire Solar System was just a vast cloud of hydrogen, helium and other trace elements. Some event, like a nearby supernova, caused the cloud to collapse through its mutual gravity. As the ball collapsed down, it started to spin because of the conservation of momentum from all the atoms in the cloud. As it spun, it flattened out into a disk. The Sun formed out of a bulge in the middle, and the planets formed in the disk.
The planet started out as nothing more than dust, but then these dust particles collided together, forming larger grains, pebbles, rocks, boulders and eventually planetoids. For the first few millions years, the Solar System was a dangerous place with these planetoids constantly crashing into one another. Life wouldn’t stand a chance to survive.
Eventually the number of objects in the Solar System was cleared out; they were either swept up into the planets, or kicked out of the Solar System by gravity. And we were left with the planets we have today.
Astronomers know that everything in the Solar System (including Venus) is roughly 4.6 billion years old through radioactive dating of meteorites. They can tell that all the meteorites in the Solar System were formed at the same time because of the percentages of radioactive elements they contain. And they’re able to determine how much of these radioactive elements have decayed over time, to determine their age.
So, how old is Venus? 4.6 billion years old, just like everything else in the Solar System.