I started DS at 4 months because he was having so much formula and still seemed hungry! He was always looking at every mouthful we were eating ourselves! He had a good grasp of objects and good head and neck control but couldn't (and still can't at nearly 6 months) sit up unaided.
I want to do baby led weaning but he won't do that until he can sit upright on his own! In the meantime I'll puree fruit and veg!
Oh and I think it just depends on the individual baby as to whether his system is ready or not, as my LO was eating 3 meals a day plus 5 bottles by 5 months and is happy as Larry!!