As noted above, baby teeth get loose and fall out when the permanent tooth is ready to grow in. In many instances, once a baby tooth falls out, the permanent tooth is already visible at the level of the gums. If the adult tooth is visible when the baby tooth falls out, it still may take 6-12 months for the permanent tooth to fully grow in.
For children whose mouths are very crowded, it can often take over a year for permanent teeth to grow in after the baby tooth falls out. The reason being, most permanent teeth are larger than baby teeth; therefore it can take more time for those larger teeth to push their way through the gums and get oriented into the correct position. For children who have insufficient space, permanent teeth may grow in crooked or rotated. The more crowded the mouth, the longer it will take for permanent teeth to grow in.