Found my good old Robin Barker's Baby Love (2005), page 356;

The optimum time to start solids changes every decade or so. In the 1920's it was nine months, in the 1970's it was six weeks, in the 1980's it was three to four months, in the 1990's it was four to six months and now it is six months.
Previous recommendations were mostly based on whim, fashion and whatever foods was around at the time; now they are mostly based on reliable research and greater scientific knowledge of nutrition, physiology and baby development (we hope). However, cynic that I am, it will be interesting to see what the story is in ten years' time.
She didn't have to wait for ten years now that experts' recommendation has changed again to 4-6 months... Very interesting, as she said, isn't it?

SJ & Marcellus,
My 4.5 month old baby has shown all the signs mentioned. He loves to chew anything in that regards!