By age 3 or 4 years old children should be able to recognize numbers and letters.
But the 3 or 4 year old may not be able to fully recognize all letters or numbers depending on the child.
Some children are smarter than others and some children are the type to enjoy learning letters and numbers.
A good thing to do is buy your child some letters and number toys such as the letter and number magnet set that they can put on the fridge or other magnetic thing or just play with it anywhere.
I would also recommend buying them an easy to read book with larger letters and pictures in them and let them try to read it by themselves.
But you can and should also help them read by reading to them daily.
That way the child will be prepared more for learning in Kindergarten.
By 5 and 6 years old the child should be able to read more on their own but all children are different and learn at different ages.