It's never too soon to start your child's journey into Mandarin
A child starts learning his mother's language while he is still in the womb and if you are trying to introduce Mandarin as an additional language, you should start your child on their journey into Mandarin as soon as possible. Like mastering his native language, your child will be able to acquire Mandarin, to imitate the adults and children around them, and to develop thinking of the world in the Mandarin context, as if he is growing up in a Mandarin speaking place, such as China or Taiwan.
In addition, children are "primed" the first three years of life with synapses at a peak, busily setting up the optimal neural pathways to mediate language. Young children who learn through play and natural interactions are more likely to become native speakers, and without an accent. An additional benefit to an early start is that Mandarin, being a tonal language, develops the brain more as it requires both sides of the brain to interact. This compares to a non-tonal language such as English, which only requires one side of the brain.