Hello. If u have pain only in the back with stiiness then its most likely a bony problem. If u have back ache with radiating pain, tingling or numbness down in ur legs then u have nerve problem which again is due to the bones around. Anyways at ur age its mostly muscular or inflammatory. Get an xray of LS spine and xray of pelvis with both hip. Get a vit D and
B12 level tested. Improve ur posture and do some stretching exercises and medication advised after consultation with an orthopaedic surgeon. Thanks.