Primary tooth eruption and exfoliation  (falling out) and eruption of the permanent teeth varies from person to person - some children don't lose the last of their primary teeth until age 14 or 15.  And, occasionally, a permanent tooth will be congenitally missing, which prevents the primary (baby) tooth from loosening. This is one reason tooth development need to be checked by radiograph (x-ray)

There are twenty primary teeth, and thirty-two permanent teeth. The permanent molars do not replace primary teeth  - they erupt behind them, starting with the first molars at about age six.