Hii!
According to the Indian curriculum, a child / student studying in class 6th should be between 11 to 12 years of age.
The age and class accordingly are-
* 1st grade - 5-6 years of age
* 2nd grade - 7-8
* 3rd grade - 8-9
* 4th grade - 9- 10
* 5th grade -  10- 11
* 6th grade - 11- 12
And so on.....
Hope this helps.
Good luck!