Took our nipper out (very very carefully) for the first time when he was about 7 months, he loved it and it sent him to sleep better than anything else we’d tried up to that point, transferring a snoozing infant from trailer to cot soon became a well honed skill.
From about ten months he was more than happy to go for longer, faster road rides and after about a year we started venturing off road. Unintentionally Got him airborne at the FOD at 13 months, he didn’t even wake up!
Mind you this is in a Chariot Cougar 1, which has pretty decent head support and suspension, without this we’d probably have waited much longer.