A child who cannot yet read the Arabic letters starts with the Noorani Qaida: the 28 letters, the vowel marks and how letters join, before moving on to the Quran itself. A child who already reads joined words starts further on, building fluency and basic Tajweed.
Readiness matters more than the exact birthday. From about 4, what counts is whether a child can sit with a teacher and repeat sounds. If you are not sure where your child is, a teacher can find the right starting point by listening to them read.