NiNoKuni - you asked about why the 'magical' 6 month mark for introducing solid foods alongside milk... The guideline comes from having better scientific knowledge now about how the human gut works.
To put it in a not-very-scientific-way, a baby's gut is porous - having little holes in it. These holes BEGIN to close from around 17-24 weeks, and the vast majority will have a 'closed gut' by 26 weeks (6 months).
If food other than milk is given before 6 months, there's a chance that these molecules pass through the holes and the baby's immune system launches an attack on them - not usually noticeably, but enough to cause potentially lifelong sensitisation / allergies / intolerances.
We can't tell from looking at a baby's behaviour whether their gut has matured, hence why it's 6 months. Whilst mums might say:
"My baby at 16 weeks just grabbed a sausage roll and stuffed it in, so they were obviously ready"
Babies at 16 weeks are likely to stuff anything in their mouths - doesn't mean it's good for them.
So reaching 6 months IN ADDITION TO other signs of readiness such as being able to sit up, the disappearance of the tongue thrust reflex etc is thought to be best.
Not writing this to judge those who ignore the advice btw - as long as your choices are informed all is dandy.