Our boys used the trailer from 18mths but the rear seat, from 9 months . As soon as they can sit up and support their weight plus, and this was the big deal, as soon as they're heads were big enough for a helmet was our deal
I think the general consensus is that they should be able to hold and support their own heads, unless they're sitting a carrier specifically designed for babies.
Our boys used the trailer from 18mths but the rear seat, from 9 months . As soon as they can sit up and support their weight plus, and this was the big deal, as soon as they're heads were big enough for a helmet was our deal