I tried at just over 3 months, as advised by HV. Baby was (still is) big for his age and gaining weight rapidly, watching us eat, trying to get hands in our dinner, drinking LOADS of milk (as in approx 55 oz of breast and formula milk a day.)
I couldn't believe it when he didn't want the mashed banana!!!!
I offered rice a couple of days later and porridge a few weeks after that, and still not interested.
It might be worth a go but if your baby doesn't want it, there is not a big pile you can do about it. I was glad in a way that my baby refused it though, as I did feel that under 4 months was a bit young.