From [0]:

Bennu's basic mineralogy and chemical nature would have been established during the first 10 million years of the Solar System's formation [...]

Bennu probably began in the inner asteroid belt as a fragment from a larger body with a diameter of 100 km. Simulations suggest a 70% chance it came from the Polana family and a 30% chance it derived from the Eulalia family. Impactors on boulders of Bennu indicate that Bennu has been in near Earth orbit (separated from the main asteroid belt) for 1–2.5 million years.

[0] https://en.wikipedia.org/wiki/101955_Bennu#Origin_and_evolut...