DS has been able to hold his head up and still from birth, so I don't go by that.

I have been able to do some searching, and it appears the gut matures between 4-6 months. This makes sense to say start solids at 6 months - at this point, baby's gut WILL be mature. At four months it probably won't. However, starting when baby tells you it's time to start between 4-6 months is the best idea.

I've always hated the idea of "on this day, set the day you were born, six months ago, you are going to have a solid meal." Why not let baby tell you? After all, we let baby tell us when it's sleep time, nappy change time and the rest, why put baby on a timetable for food?

I have a good leaflet from La Leche League, saying when baby is ready to start solids. Here are the two first babies telling you their needs:

I am still exclusively breastfed. Now I am starting to sit up unsupported. I can grab things and bring them to my mouth. I no longer push things straight out of my mouth with my tongue and my digestive system is maturing.

I can sit up well. I am being offered tastes of single foods. I like to pick up suitable finger foods. I like to explore them with my mouth and hands. Almost all of my calories come from breastmilk.
DS has been able to put a spoon full of food in his mouth before we started solids; he loves his rusks when he has them and is able to feed himself (a real bonus when I'm hungry!). Just because he isn't six months doesn't mean he isn't ready for solids - he would let me know if he wasn't.