Wait until your baby sits unsupported for ten minutes, usually around 9-12...

https://devinmqjy235.iamarrows.com/what-weight-limit-matters-more-the-child-seat-or-the-rear-rack

Wait until your baby sits unsupported for ten minutes, usually around 9-12 months. Front seats keep them close, rear seats suit active toddlers, and trailers offer the most stability. Ensure they hold their head up well. You’ve got this!