Can the sacral vertebrae flex and extend?

Answer 1

Not really as they are fused into what is effectively one bone.

______________________

Answer by Puhlaa:

While a child the sacral vertebrae are joined by disks similar to those of the rest of the spine, giving the sacrum some flexibility. This becomes especially important for a child while learning to walk, as frequent falls on the bum could fracture a solid sacrum/coccyx. The sacral vertebrae begin to fuse at around 18 years of age, and are completely fused by 25-30 years of age. Thus, the above answer would indeed be correct if in reference to an adult older than 30. Otherwise, if in reference to a person under 25, indeed the sacral vertebrae would allow for some limited flexion and extension.