My mother is in her 90's and her cardiologist said he heard a bruit in her carotid artery for the first time. is this significant at this age?

A carotid bruit is: significant at any age. The question is whether you will want to do anything about it at that age. If she has not had a stroke or any symptoms you will probably do nothing. Maybe additional diagnostic evaluation is warranted with ultrasound but surgery is probably not a consideration. Options for additional evaluation as well as additional intervention should be discussed with her health care.