Developmental Dysplasia of the Hip, a Slipped Capital Femoral Epiphysis and Perthes Disease might cause arthritis of the adult hip (ie, later in life). Why might successfully treated Developmental Dysplasia of the Hip have the best prognosis whereas untreated Developmental Dysplasia of the Hip might present the biggest treatment (surgical reconstruction) challenges?
DDH is truly treatable: relocate the hip before it deforms (or rather, "fails to form correctly"), and that maneuver is tantamount to a cure.
SCFE pinning is basically halting progression: make it better by not allowing it to get worse.
The "Treatment" of Perthes is lots of prayer: that is, the patient may get well even without treatment (or fail despite it).
At the other extreme, untreated DDH is the worst, because not only do you have bad arthritis (as you might with the other two, too-) but you don't even have normal anatomy with which to reconstruct the hip! There is no socket either: the pelvis, as well needs normal anatomy on the other side of the joint, ie femoral head, to form correctly. If either is missing, then the other side of the joint can be marred.
Try to recreate these from memory to underscore your understanding of these conditions. As shown, the acetabulum (the "socket" for the ball of the femoral head) is shallow, possibly to the point of non-existence, in DDH