Wait until your baby sits unassisted and holds their head up steady, typically...

https://gunnerawsh736.cavandoragh.org/is-it-safe-to-bike-with-a-baby-if-they-were-born-premature-a-guide-for-parents

Wait until your baby sits unassisted and holds their head up steady, typically around twelve months. Strong neck muscles matter because helmets add weight that shifts during bumps