I have briefly touched on the this topic in Graduating to Solids .   To recap, delaying the introduction of solids to babies until at least 6 months old is  in line with WHO and the AAP, as well as our own Health Promotion Board's recommendation of breastfeeding exclusively for at least 6 months, i.e. no other food except breastmilk.

If you are not breastfeeding, it does not mean that the 6 months rule does not apply to you.  Delaying solids until at least 6 months protects the baby against diseases and allergies.  You can read more about it here. Hence, for health reasons, even though your baby is formula fed, it is still advisable to delay solids until 6 months.  Even if your baby seems to be exhibiting all the signs of readiness, it pays to delay solids until 6 months old because of all the health concerns.

I mentioned in Graduating to Solids about the readiness of the baby and how delaying solids actually make introduction of solids easier on you and your baby.  Our third baby did not start solids until 8 months old!  It was not because we didn't want to start him on it but by 5 months plus, the boy was not even showing any interest in food!  At 6 months, we tried to introduce solids but he only frowned like we gave him lemon to eat and spat the food out.  After a few tries, we concluded that he was not ready to eat yet and waited another month.  At 7 months, we tried introducing solids to him again.  Although this time he didn't spit out the food, he wasn't very keen on eating either.  Once again, after a few frustrating tries, we decided to wait again.  The boy only really started eating at 8 months.  Even then, he was unlike his brothers and ate very small portions.  Honestly, it did take me a while to get used to the idea that he was just a small eater.  You have to remember I had two good eaters before him so he was a bit of a 'culture shock' for me.