Generally, if a child got his first tooth at an early age as a baby, he will loose it at an earlier age as well. If he got his first tooth a little late, then he will loose his first tooth later than most.
The typical age to loose the first baby tooth is 6 years of age. About 90% of kids loose their first tooth at age five or six.
The first tooth to be lost is almost always one of the mandibular (bottom) front central incisors.
If you look at a chart of tooth exfoliation from the ADA, you will see they list the lower front central incisor as falling out around 6 to 7 years of age. That's pretty normal, however, I see lots of 5 year old kids with their first loose tooth. If I'm making a chart, I'm putting 5-6 rather than 6-7. I see just a few children loosing their first tooth at age 4, and there are many that do not loose a tooth till age seven. There are a few loosing a tooth age eight, but that's definitely on the late side. They key is that it varies a lot. Four is definitely early, seven or above is later than most, but it's all normal. If you are not sure, ask your pediatric dentist if your child's eruption pattern is right on track. Also, some baby teeth do not fall out till age 12 on average.
There are a few syndromes or medical reasons for delayed eruption, but that's pretty rare.
See here for something that happens often:
Permanent tooth coming in behind baby teeth
In case you were wondering , the first baby tooth usually comes in any where from 4 to 12 months of age; usually 6 months of age is average. This varies a lot as well.