Answer:
What Paul meant to say about the believers in Corinth when he used the metaphor about “milk” and “solid food”, was that when the believers in Corinth were growing up as spiritual babies, he gave them the basic teachings, which are easier to assimilate, and which he indirectly referred to as milk: every baby is given milk, but would grow and become capable of eating solid food. At the time, the believers were not yet matured for solid food: by using “solid food”, Paul meant to say that there are bigger teachings or lessons which are easier to assimilate after one grows spiritually, and is no longer a baby.