Every one develops at a slightly different rate to other people. That's true whether it's learning to walk or talk, losing your baby teeth, reaching puberty or whatever.
It also applies to the development of intelligence.
Many things affect how your brain develops. It is possible that it is partly related to inheritance. That is, some people may take longer to develop in certain ways (to understand hard maths or to sing in tune) because their parents took a long time.
Another thing is definitely true. The more you use your brain (up to a point) the better it will work for you.
So, if you have the right attitude, either way you're going to keep on getting more intelligent.