For the first year an infants PRIMARY source of nutrition should be breastmilk or formula.
That said;
All babies digestive systems develop differently. There's no magic number. Heck, I've known one baby who were started on pulverized adult food at 2 weeks because the mum lost her milk and they were stuck in Uganda (fortunately they got to a village with nursing mothers and the baby was able to be wet nursed). Tasting (or practice as another poster said :) can really start any time. The most common is around 6 months. Some babies can digest food at 6 months, others it causes tremendous gas/colic/diarrhea/vomiting. For most it's somewhere in between.
If you're on this board long enough you'll notice something. A HUGE number of requests dealing with digestive problems around 6-7 months. The old-school of thought was that introducing foods is just that...introducing. A lick here. A taste there. Cheerios more for motor practice that anything else. Not really expecting any kind of nutritional benefit until approaching or after the first year.
Lately though, you'll see a trend of women actually trying to do "meals" with their infants. Mostly it doesn't work (digestive problems, sleep problems...there's not enough calories or fats in babyfood for growing babies). For some it does work. Babies are ALL different, or there wouldn't be milestone ranges, sleep problems, height/weight charts, eating issues, etc. We all very quickly become experts in our own :) but that doesn't translate even to a neice or nephew...much less outside our genetic pool. A child who was born at 10 lbs 23 inches and is 30lbs & 32 inches at a year from a family that's all over 6 feet, is going to have vastly different nutritional needs/patterns than a 6 lb, 17 inch baby who is only 15 lbs 22 inches at a year from a petit family.
Mostly it has to do with paying attention to your own child, seeing how they react, and not pushing an agenda unless (like my friend in Uganda), it's an emergency. Remember, regardless of when you introduce solids; for the first year an infants PRIMARY source of nutrition should be breastmilk or formula.
The signs are licking their lips, staring at you eat, trying to taste your food, and a happy or quizical reaction when you do let them try it (as opposed to scrunching up their faces, trying to spit it out, vomitting/colic/etc later in the day, or any sign of allergic reaction).