It depends on what country you are referring to. In the U.S. you can usually add 5 to the grade to get the approximate age. Kindergarten would be grade 0. 0+ 5= 5yrs old. 1st grade, 1+5 = 6 yrs old. Senior year is grade 12. 12 + 5 = 17.
Depending on how early or late in the year can have an impact on the age they start. I graduated at 17, and most of my friends were 18.
There is also a new mandatory remedial at 3rd grade if students don’t read at or above the 3rd grade level by the end of their 3rd year, so it may be more probable to add 6 after the third grade.
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