It all comes down to the indivdual. Personally I would recommend starting no younger than the age of 12 as your hands need to be big enough to play chord shapes. If you are younger and are keen to start playing I would buy a 1/2 sized or 3/4 sized guitar then, learn from a method book such as "Hal Leonard Guitar Method 1". In this book you will learn notation and be able to play simple melodies. Once you get a little older you can introduce chord shapes and strumming patterns . The other option would be to start on a ukulele as you can start learning chord shapes at a younger age to prepare you for the guitar.