* Legally: In most places, the youngest age a child can be to start kindergarten is 5 years old. So, the oldest person in kindergarten would be almost 6 years old.
* Realistically: It's possible for some children to be slightly older if they had a late birthday or were held back a year.
* The "oldest" could also mean the child who has been attending kindergarten the longest. This might be a student who is repeating the grade.
So, it depends on what you mean by "oldest"!