I was just wondering the same thing. At what age does one take these classes?
Where I'm at in the U.S. Calculus is usually a very late/advanced high school class or an early college class. Basically high school offers AP Calculus A & B (which if you pass the AP test can count for college credit in Calc I & II respectively) that the top math kids take. Even most top students graduate after AP Calc A with only a few being advanced enough to go on to Calc B (Arizona usually only has like 5-25 Calc B high school students taking the test each year). Early college then offers Calculus I, II, and III as things you would take as a freshman/sophomore in most fields that
need it or later on in less mathy fields.
Basically the "best" math progression plan (as in you don't fail, need to retake things, or spend semesters doing nothing) around where I live in the U.S. goes like this:
X-grade math until 6-8th grade
Algebra - 8th
Geometry - 9th
Algebra II - 10th
College Algebra/Pre-Calculus - 11th (College algebra often counts for dual credit so you can walk into college having that done already if you need it)
AP Calc A / College Algebra / Pre-Calculus - 12th grade (Depending on what you've taken before)
Calculus I - Freshman 1st semester
Calculus II - Freshman 2nd semester (if needed)
Calculus III - Sophomore 1st semester (if needed)
Whatever "higher" math classes your college degree requires
It's pretty common for people to be "behind" by 1 semester, finishing 12th grade by taking college algebra or not taking math at all, not taking Calculus I until their first semester in college and there can sometimes be some interplay between the order you take Geometry and Algebra II in, but that's the basic order.
Edit: Oops, forgot Pre-Calc, updated