If after various antibiotics treatments, the uti still persists, should a 70+ yold be very concerned? ultrasound done and no stones found.

Maybe: Depends if you are symptomatic. Some older people, usually females, may have something called asymptomatic bacteriuria (urine tests performed by medical doctor may show an infection however the patient does not have bothersome urinary symptoms such as burning, frequency, pain). Treatement for patients with "dirty" urine tests that appear infected without symptoms is generally not recommended.

Asymp bacturia : If you are not having any symptoms and the ultrasound showed healthy kidneys, no stones and an empty bladder after voiding then you can safely stop checking the urine altogether. Many women after menopause have asymptomatic bacturia (bacteria in the urine). These bacteria are often normal colonizers like the bacteria in the mouth or the gut. If there are symptoms a urologist could help further.