Can bow legs get worse with time?

Yes worsening occurs. Bowed legs (varus knee) can worsen with time in that the varus alignemnt can advance about the knee with advancing collapse across the medial (inside knee joint space). Any medial bony lesion or deformity theoretically can worsen, thus worsening the varus alignment.
Deformity. Bow legging can be a sign for early vit d deficiency as well as other associated bony defects. But if it chronically persists and left untreated it can be devastating.