* Birthdate: School systems typically use a cutoff date (e.g., September 1st) to determine grade placement. A child born just before the cutoff will be in a higher grade than one born just after, even if they're only a few days apart in age.
* School system: Different countries, states, and even individual school districts may have slightly different grade structures or age ranges for each grade. Some might have kindergarten, others might start with first grade. Some systems might have different age ranges for certain grades.
* Grade retention or acceleration: Students who repeat a grade or are advanced to a higher grade will be out of sync with their age peers.
* Education system: Some educational systems may have different structures altogether (e.g., some countries use different numbering systems for school grades).
To know what grade someone *should* be in at a specific age, you need to know their birthdate and the specific school system they attend. Even then, individual circumstances might mean they're not in the "expected" grade for their age.