Assuming the child is neurotypical, home environment is a huge factor in early language development. A child who grows up in a home where poor grammar and language are used will usually start school with poor language skills, meaning the teacher must overcome a tremendous deficit from the get-go.
My parents insisted I use correct grammar, even before I started to school. As a result, I started first grade with above grade level language and comprehension skills because I had been exposed to proper usage since I was born. It is a gift that has served me well my whole life and I am so grateful for it.