I started at the age of 10 with HTML, around my 14th i began with C++, so i dont think there is a real age to start with coding, depends on ur IQ and ur will to learn,
Also there isnt realy a time period for getting to know it, its different for everybody, also -> you can never be done with learning, just learn the basics first, then expand ur knowledge, if there's anything i can help you with, let me know..