At what age does solid food, not milk, become more vital for a baby? When can I stop worrying that my baby is getting enough ounces of milk?

Teeth. I believe our body tells us what we need. When teeth come in it is time to start bringing food in. Breast milk is fine but i tell my new moms that when a child has teeth to chew 9 to 10 months is a good time to start bringing in rice and other vegetables. But only bring in 1 every week or 2 so she can watch for adverse reactions. Breast feeding the first 9-12 months is the best option.
Weaning. If breast fed you can continue but you can start cereal as early as 3-4 months and gradually add more canned baby foods through the first year. When he/she has teeth, you can start junior foods.