My lower back hurts all the time. My parents say it's from stress. Ae they right?

Not typically stress. Chronic low back pain of > 3-6 months is not likely from stress. Certainly musculoskeletal causes including sitting position and posture may contribute but a more serious issue is also possible including disc pain and fracture. Long standing pain should be evaluated with history, physical examination and possibly xray and MRI if warranted.
Lower Back Pain. Lower back pain can be from multiple etiologies. The number one cause of chronic lower back pain is muscle imbalance. This can be improved by proprioceptive training of the muscles by performing balance exercises. Other causes are types of dysfunctions known in the osteopathic world as "somatic dysfunction" and can be corrected by an osteopathic physician.