After 4 months. If a baby can coordinate the swallowing without choking or having everything pushed out by the tongue, it's probably ok to spoon feed cereal. Most babies push some of the food out at this age, and with baby foods they need to try at least 5 times to get used to the taste (or not). Patience is key.
No set age. I usually recommend starting foods at around 4 months. Some start earlier. It takes a time of development for a baby to have a good oral swallowing mechanism for more solid foods. Some thicken formula with cereal at 6 weeks. I don't ususally recommend this. One could wait much later to start foods outside of formula. See when you think your baby is ready. Don't force the issue. Enjoy!
I wait until sitting. Studies of various populations show that breast milk alone supplies balanced nutrition for the first year +. Most early solid feeding pressure comes from the days babies were fed home made blends of boiled cow milk/karo & vitamins.At the time early supplements made sense, now they are overdone.Starting should focus on training baby to sit up & eat in preparation for his/her position at the table.