This is true, the start date for kids to have enough head power for that is 1 yo and its not a much the helmet as the bumps and forces a bike puts on their head.

We started at 1 YO and she was almost exclusively in the bike seat. Now my LO is 2 and quiet comfortable in the bike seat or the trailer although she prefers the trailer, mostly because she can play with her toys in it while I haul her around. Also if she falls asleep, I can let her finish her nap where the bike seat it too unstable without me on the bike.