Children can usually begin recognizing some letters by age 2 years old.
By age 3 the child starts learning more of the letters of the Alphabet and by age 4 years old the child begins to connect some of the letters with words when reading.
By age 5 the child should learn and know most if not all of their ABC's in the alphabet.
My oldest son surprisingly knew all the letters in the alphabet by age 4 years old and was reading really good by age 5 when he entered Kindergarten.
He was and is very smart.
His younger brother though didn't fully know his ABC's until age 5 years old and then was reading much better by age 6.
All kids are different and there's no set age on when they will know their ABC's but most kids will know their full ABC's by age 5 or 6 and be able to read.