How old does a baby have to be?......

[daisiesinaline]

How old does a baby have to be before they can go in a child seat on the back of a bicycle?

Not sure what the legal age is, I would imagine they would have to be old enough to be a very good 'sitter' IYSWIM, to cope with the swaying from side to side

You can get dutch seats that are good from birth, that go on the front of the bike. But all the seats (and helmets) I've seen here in the UK are from 6 months. The child should be able to sit up on their own. I'd probably cheat and put a 5-month-old on the back of a bike, in a reclining seat, for short periods.