This may be an old chesnut...please forgive me if so!
DD was born a month early, although you certainly could not tell now, since she is 91st centile for weight and hitting (non adjusted) milestones just fine. I BLW my older boys (well, DS2 just started swiping food off my plate) typically at around the 5.5 month mark, because they were so obviously interested and ready.
On the one hand I am reluctant to start DD at this stage, since technically her little guts are only 4 months old. On the other, she is well capable of sitting up, with a little support, in an appropriate chair, her grasp is adequate for getting stuck in and she is giving 'tractor beam' glares whenever she sees anyone eating - trying to pull the food in through sheer will power.
So my question is how much longer should I wait? Shall I torture her a little longer and make her wait until she turns 6 months? I haven't had her weighed recently, but she's not showing signs of hunger that breastfeeding isn't satisfying...so no clue there, I'm afraid.