A critical study investigating MRIs found that "normal" patients, patients with no symptoms of back pain, often have "abnormal" MRIs. In fact, the study found that disc degeneration or bulging is seen in at least one third of patients in their 20s and 30s. In patients over 60 years old, disc degeneration or bulging is seen in almost every patient studied...and these are patients with no back pain.
This is not to say that MRIs are not helpful, but just because an MRI is "abnormal" does not mean that there is necessarily a problem. As people age, disc degeneration and disc bulging are normal findings. Just as people find a few grey hairs, or a few skin wrinkles, the spinal discs show signs of aging, often as early as people's 20s and 30s. Determining if an abnormal finding is a cause of your back pain depends on your symptoms and examination findings.
Sources: Boden, SD, et al. "Abnormal magnetic-resonance scans of the lumbar spine in asymptomatic subjects. A prospective investigation" J Bone Joint Surg Am. 1990 Mar;72(3):403-8.