What are the way to assess IQ and mental development of child?

Hello doctor,

Kindly help me to understand my elder daughter's behaviour. She was born 14 years back. At her childhood, she was very energetic and hyperactive. We had put her in pre-school at 2.5 years and now she is in class 9. She is 13.5 years old now. I am finding her maturity level is far below her classmates and she hardly have any friends. She is also very poor in study. We are in such a condition that we only expect her to pass the examination. Sometimes I am feeling guilty about the fact that she is always under pressure.

She nowadays does go out and keep herself busy with mobile and TV (cartoon). I have another daughter, who is 6.5 years old and the elder one cannot get along with her sister most of the time. She always tries to attract the attention by some means or other. She is only growing physically, but I am finding the mental growth is not at par. Kindly help me.