Are there separate IQ tests for different age groups?

Yes there are separate IQ tests for different age groups. Very young children will be given a different I.Q. test than a child of school age, the younger the child the less reliable the I.Q. test.

Once entering adulthood (16+) another test will be used.

The scores one gets on a test are not directly comparable to people of a different age, for example: If a 16 year old got an I.Q. test score of 110, and a 25 year old also scored 110, this does not mean they scored equally on the test.

If the 25 year old were to have answered all questions exactly as the 16 year old did, they would have gotten a much lower score than the 16 year old. This is because, even in the same test, people are compared to others of the same age group, not all takers of the test.

The mostly commonly used tests for school aged children is the WISC (Wechsler Intelligence scale for children) and for adults it is the WAIS (Wechsler Adult Intelligence Scale).