Hi, this is Dr Hitin Mathur, Orthopaedics surgeon at Fortis C Doc Hospital, new Delhi. At your age back pain with radiation to ankle is a sign of bone weakening along with nerve compression in the back. A good examination of the back and leg would be helpful. Sometimes low vitamin D can cause back ache, so it would be wise to get that tested. Learn spinal extension exercises from a physiotherapist and do them 20 mins 3 times a day. Also get x ray LS spine AP and lateral views on empty stomach. Start some analgesics for the time being and see me or an orthopaedic surgeon in your area to find the cause of the pain. Avoid prolonged sitting, forward bending and weight lifting.
Low back pain and radiating to your legs is usually due to lumbar spine problem.. It is usullay associated with numbness and weakness..so consult a neurosurgeon and have a complete neurological examination and follow his advice