Most children are capable of brushing their own teeth by age 6 years of age.
However some children may be able to brush their own teeth by age 4 or 5 depending on the child but they should always be supervised while brushing their teeth until at least age 7 or 8 years old.
My 2 year old boy can brush his teeth by himself after I put toothpaste on his toothbrush although he brushes his teeth himself I have to make sure he rinses his mouth out of the toothpaste.
I was brushing my own teeth by age 6 years old but my mom helped me sometimes to make sure I got my teeth clean.
She also helped me floss until I was 7 years old.