Yes perthes disease onset at late age after 6 years of age are difficult to treat but leaving it without any intervention is wrong. what happen in this disease is that the joint between hip and upper end of thigh bone is affected in this due to poor blood supply. in its acute stage if pressure is kept on that limb then more blood supply will be hampered and may result in permanent disability which can be corrected surgically. if in its early phase if that limb is given rest then the illness can be reversed , by giving rest blood flow to that area will be regained by new vessels. In lower age groups bone remodelling can be better achieved so the treatment in later age group is difficult but treatable if shown to good pediatric orthopedics surgeon. giving shoe correction for balancing the weight equal to both hips is a conservative method. correction after age of 18 told to you is said because by this age bones grow to the adult size and remodelling can still be expected at this age but is usually slow. In my opinion please take opinion of a gud orthopedic surgeon and please share with us