It was 6 months when I was a baby, so it's been around for at least 22 years!
I personally don't think there really is an exact "age" to start... DS has only gotten into his solids the last month, others love it at 4 months or less...

IMO i believe people wrongly think their babies are ready for food because the watch them eat with interest, they grab at food and cutlery, they stop sleeping as well etc etc.. all of which are very normal developmental leaps, kids watch what we do becasue its how they learn, babies watch us go to the loo, does it mean they're ready to toilet train..??
totally agree. and i'm not affraid to say that I tried ds on some solids just before he hit six months just because of all those reasons ^^;