Why are moon rocks younger than most meteorites?

The most meteoite material that people have dated stems from the time when the solar system formed and stems from bodies that never managed to grow the size of planets.

When people date materials that are that old, they use techniques that tell them when the material solidified for the last time either from a homogenous protoplanetary nebula or from a liquid magma.

Since the moon is believed to have formed by a giant impact of a roughly mars-sized body with the proto-Earth millions of years after most meteorites were formed and moon experienced ongoing magmatic activity thereafter, the ages obtained by radiometric dating of moon rocks are usually younger than those of meteorites.