The short answer
Most children begin formal phonics between 4 and 5. Sound play can start much earlier, around 3. Some children are ready at 3½ and some not until 5½, and within that range the difference says very little about how well they will eventually read.
What must come first
Phonological awareness — hearing that words are made of sounds — comes before letters. A child who cannot hear that cat and car start the same way will not get far with the letter c, because the letter is meaningless without the sound it maps to.
This is entirely an ear skill and needs no print at all: rhyming, clapping syllables, spotting the first sound in a word, playing 'I spy with my little eye, something beginning with /s/'.
Signs of readiness
Look for these rather than an age: your child enjoys rhymes and notices when words sound alike; they can hear the first sound in a word; they show interest in print and ask what words say; they can focus on a shared activity for five to ten minutes.
Three of the four is plenty. Waiting for all four is waiting too long.
Starting too early costs more than starting late
A child pushed into formal phonics before they are ready often concludes that reading is something they are bad at — and that belief is far harder to undo than a few months of lost time. A child who starts at 5½ with enthusiasm typically overtakes one who started at 4 with dread, within a year.
