Why do knees hurt when you start squatting, and can you fix it?

Usually it is load or technique rather than damage. Common causes: increasing weight faster than tendons adapt, letting the knees collapse inward, doing everything on hard floors with no progression, or simply having weak quads and glutes for the load. Things that generally help are slowing the descent, controlling knee position over the middle of the foot, reducing depth temporarily to a pain-free range, and building up quad strength with split squats and step-ups. Pain that is sharp, swelling, locking or giving way is a different category and needs assessment rather than adjustment.