I know the sponsors of TOSRV, won't let a child under two years old on their rides citing safety concerns.
Speaking from experience, I disagree. Our daughter was in LAW (now League of American Bicyclists)Bulletin on a Christmas ride secure and snug in her carseat riding in a Cannondale Bugger in 1976.
I rode up until two days before she was born, and she started riding at about 2 months and cross country skiing with us at three months.
We adapted our Nordic Trak trailer for our granddaughter and her carseat. She didn't ride in it a lot, but it worked extremely successfully. She now at 23 months puts on her own helmet, and climbs in. We don't use the carseat in the trailer anymore.
Last week, my daughter had her in the trailer and heard a large vehicle behind her, it was a fire truck. One of the firefighters ran up and checked to make sure Annie was in a helmet, and told my daughter they would have given her one if she wasn't wearing a helmet. Poor Jeni thought maybe she was delaying them. I think they were watching Jeni, because she was exhausted when she came home do to a rigoous ride.
Our son and daughter both rode in the trailer together as they got older.