When I bend down to something up I need to use my hands to get back up, why is my lower back so weak?

You need to . Strengthen your core musculature. Usually an active and regular exercise program will help along with weight control and avoiding smoking if you do.
Depends. Some have this problem by becoming inactive, getting out of shape. You may be able to recover by doing exercises to strengthen the abdominal (tummy) muscles and back muscles. Your primary doctor can refer you to a physical therapist who can train in the these exercises.