There is no minimum age for any course, but here are some guidelines: If you are getting stuck, try doing the previous course. If you are getting stuck on course 1, complete the lessons on "Learn C++" even if you already know C++. If "Learn C++" is too complicated, then try again in a year.
If you would like to see substantial progress, expect to put in several hours a week.
Learn coding syntax to write your first program!
(USACO Bronze and Codeforces Div 2 A/B Curriculum)
If you can write simple programs (using tools like loops), learn algorithms and data structures.
(USACO Silver/Gold and Codeforces Div 2 B/C/D Curriculum)
Once you know all the standard algorithms and data structures, apply them to hard problems to score higher in programming competitions.
(USACO Platinum and Codeforces Div 2 D/E , Div 1 D curriculum)