The age of a child in 7th grade will depend on the age they started kindergarten, and whether or not they skipped any grades or were retained to repeat a grade. For example, if a child is age 5 in kindergarten and progresses through all grades without skipping a grade or being retained for any of the academic years, the child will be 12 in 7th grade (age 6 in 1st grade, 7 in 2nd grade, 8 in 3rd grade, 9 in 4th grade, 10 in 5th grade, 11 in 6th grade and 12 in 7th grade). Some children will experience their birthday during the school year, and while starting 7th grade at age 12, may be 13 by the end of the school year.
I'm including a link below to information about some developmental, academic and social milestones for 7th grade. Thanks for asking.