the US American system (Grade K-12):
Preschool 3-4 yrs old
Kindergarten (Grade K, 5 yrs old, most of the time taken at the local elementary school)
Elementary school (Grade 1-5, 6-11 yrs old)
Middle school (Grade 6-8, 12-14 yrs old)
High school (Grade 9-12, 15-18 yrs old)
(Note: compiled based on info from Yahoo! Answers)
the typical Singapore system:
Preschool 3-4 yrs old
Kindergarten (5-6 yrs old, equiv. Grade K-1)
Primary school (7-12 yrs old, equiv Grade 2-7)
Secondary school (13-16 yrs old, equiv Grade 8-11)
Junior college (17-18 yrs old, equiv Grade 12-13)
the China system:
Preschool 3-4 yrs old
Kindergarten (5-6 yrs old, equiv. Grade K-1)
Elementary school (7-12 yrs old, equiv Grade 2-7)
Middle school (13-15 yrs old, equiv Grade 8-10)
High school (16-18 yrs old, equiv Grade 11-13)
(Note: to be confirmed :P)