he has reached all his other milestones within the right age bracket.
Whilst exciting for parents, getting a first tooth (or getting the final milk teeth, or any in between) is NOT a developmental milestone. Milestones are when a baby learns to do something new (like rolling over, crawling, etc). Getting a tooth does not fit this category.
Most (read MOST, not ALL, as demonstrated by a PP) babies get their first tooth somewhere between 8 weeks old, and 16 months old. The age brackets given both for natural physiological development, and developmental milestones are averages only. If you remember from high school maths, an average is calculated by adding all the different values within the normal range, and dividing it by the total number of values used in the calculation. So this means that "average" is some babies get teeth really early, some get them a lot later (but still normal), and the rest get them in between. Somewhere, some "expert" has taken a range of ages that a bunch of "normal" babies have got their first teeth at, added them up, divided it by the number of babies in their study group, and come up with an average. HOWEVER, very few of those babies, if any, would have actually got their first tooth at the exact age that the calculation "predicts". In other words, "average" is for the most part completely meaningless. It is an impersonal statistic that knows NOTHING about your individual baby, their environment, their genetics, etc, etc, etc. All they do is make parents unduly proud if their baby performs to the average, or make parents unduly worried if they don't!