DS1 5.5months. DS2 4.5 months. The 6 month figure is because many the mothers of many babies in developing world do not have access to hygienic cooking conditions, so EBF is recommended. Personally I do not believe that every baby has some "magic switch" which gets turned on at 6 months which means that they're "ready".
DS2 stopped putting on weight, stopped sleeping through and was clearly desperate to eat (and was hugely enthusiastic once we started). So use 6 months as a guide, and certainly don't start before 4 months unless specifically advised by a healthcare professional, but between 4-6 mo ths, use your own maternal instincts and judgement.