Why does my lower back feel very tight?

Back tightness: At your age I doubt that arthritis is the cause. Muscular tightness may be due to poor mattress, poor posture sitting or walking, weak abdominal wall muscles, chronic low back strain, improper exercises and structural spinal abnormalities or disc disease are less likely possibilities. A physical trainer could instruct you in proper exercises and stretching techniques. See an orthopedic specialist