We let him play with food from around 5 months (doing BLW) but didn't really get into it until he was 6 months.
As far as Dr Sears' advice, I don't find a lot of fault with it, except there's no disclaimer for ' not seeming satisfied with milk, except when a growth spurt is responsible' because they never seem satisfied during growth spurts, so hungry at 3 months? Hungry at 4? A lot of people find that if they just increase the milk intake for a couple weeks, if that, that their LO's settle down again without the need for solids.
I know, each to their own, mummy knows best etc etc, and I can respect everyone else's decisions while also respectfully disagreeing with it, but i have yet to find a convincing argument for needing to wean before 17 weeks at the earliest, and still hold that given current research and advancements in what we know now, that it's better to wait until as close to 6 months as you can, than to not.