Learning is a cumulative process. Children cannot learn to speak without learning to understand. Without learning to speak, children cannot learn the alphabet. Without knowing the letters of the alphabet, children cannot use them to form words, etc.
This means that children start their developmental process towards reading from the earliest age. The foundation of their literacy skills starts with their cognitive and speech development. Everything else follows from there.
All children are different and develop at their own pace. The parents play a crucial role in this development by giving the child the opportunity to advance when she is ready. If the child is not ready to learn the alphabet, then she will not learn the alphabet. However, if the child is ready to learn the alphabet, she still will not learn it unless her parents give her the opportunity to learn it. The role parents can play in the early years is not to push the child beyond her abilities but to give her the opportunity to reach her abilities.
Primary school education in most countries starts at age six. From this age, schools aim to teach children to read within 3 years so that children should have a solid reading base by the age of nine. However, it is possible for children to learn to read much earlier than this.
We will split the skills needed to learn to read into three developmental timelines. These timelines greatly overlap but it helps to compartmentalise them to consider the age ranges when they can happen. The three steps are speech, learning the alphabet and learning to read.
Children start identifying the sounds around them from the earliest age. They may say their first word as early as 6-months old or as late as a year and a half. By the time a child is two years old she may have a vocabulary of 100 words. At age three, she may have a vocabulary of 500 words and by the time she is five years old she may know a whopping 5,000 words or more. Speech is the essential base from which reading and writing skills will develop.
Children do not develop the cognitive ability to recognise the complex shapes of letters until after the age of two. Before that age, they may be able to identify simple letter shapes such as 'O' or 'X'. The more complicated letter shapes will be beyond them. Sometime between the ages two to four the child should develop the ability to learn the alphabet.
Most children learn to read at six years old and become competent readers by around age eight. However, many children are actually capable of learning to read much earlier.
A few children are ready to start combining letters and identifying words as early as three years old. More usually, a child will start developing these abilities from four to six years old.
A very small number of children, usually girls, learn to read by osmosis. That is if you read to them enough, they start to recognise and identify the words without anyone ever formally teaching them. This is an unusual talent so do not be disappointed if it does not happen with your child.