Six & A Half Years Old
Usually at about 6.6 the first permanent molars erupt behind the four back ends of the baby teeth and the transition is upon them. The roots are being reabsorbed and their deciduous teeth will loosen and fall out. This begins now and continues, ending at age 12 or 13.
The appearance of the molars is the most obvious physical sign of the full transformation happening. There will be 32 permanent teeth altogether, though more transparent and yellowish than their baby teeth.