It is not unusual for some children to show a maturity that is beyond their years, and this is not something to get worried about. A parent should only be worried when a child always sits alone, and never tries to mingle with anyone.
All children are not alike. It is natural for them to inherit the traits of their parents. If a mother is serious by temperament, a child may very well take after her. Likewise, the environment in which children grow up too has a role to play in determining their character. This will be reflected in the way they play, talk and behave.
The measure of intelligence is Intelligence Quotient (I.Q). It is possible the child with higher IQ levels for this age group look serious, and expresses higher level of thinking and action.
Parents and teachers can keep certain things in mind while dealing with such children. It is always good to reciprocate their mature ways, keeping their higher intellectual levels in mind. If they come to you asking questions that are ‘too big’ for a child, don’t make comparisons with other children, and make your child feel isolated. This could make them more introverted.
Try to find out he kind of books, toys, music, etc, they like or demand. A parent of a mature child should try to become a good friend to the child. Otherwise, out-of-the- normal mental growth could lead the child to harmful things.