8 years old. But the youngest kids with birthdays after school starts but before the cut-off date may still be 7.
11th or 12th grade
A 14-year-old is typically in 9th grade in the United States.
15 or 16 years old it depends on your birthday
8 years old
a 13 year old would normally be in 7th or 8th grade depending on whether or not they were born before September.
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18 18
9:30
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It varies depending on what age you start puberty.
5 year olds are either still in preschool, or in kindergarten.
a 10 year old would usually be in 4th or 5th grade in the United States. (you're supposed to turn 10 sometime during 4th grade or the summer after, and then also be 10 when you begin 5th grade)