Many high school students have access to CollegeBoard's AP Computer Science Principles and AP Computer Science A programs, which expose students to fundamentals of computing and Java respectively (Principles is more focused on the _concepts_ of programming–sequential instructions, working in teams, etc., while CS A is focused on writing working Java). I've worked with students who do not have access to these classes at their school as well, working with them through a self-study curriculum.
Occasionally, I'll also see middle school students (6-8 grade, age 11-14) who have (or whose parents have) expressed interest in learning to code. I usually work with these students in Scratch then Python, depending on their skill levels, as some schools are even teaching programming classes.
I also get some requests from elementary students, but generally decline those.