New dentist recommends bone graft after extraction of partially erupted lower left wisdom tooth. I am 31. Is graft necessary?

Can't see. Can't see the area. Sometimes partially erupted wisdom teeth can cause severe bone loss. Sounds like your Dentist is being prudent. If you have any misgivings, however, get a second opinion.
Reasoning. What is the reasoning for the bone graft? If it is because there is a bony defect to the distal of tooth # 31, then by all means have the graft done at the time of the extraction. Let your dentist explain his/her assessment of your mouth.