Age of infant for bike trailer?

[Eden]

No kids here so definitely confirm this...... but I've always heard that they have to be strong enough to hold their head up while wearing a baby helmet.

I found this on a helmet site
Are there helmets for infants?
Yes. Many infant-sized helmets are of the soft-shell variety. They are light, an important consideration for small children whose necks may not be strong enough to comfortably hold a hard-shell helmet. Babies younger than 1 year have relatively weak neck structure. Neither helmets nor bike traveling is recommended for them

There's a whole bunch of good info here http://www.helmets.org/little1s.htm
including the pros and cons of sidecar style trailers.

[xeney]

Be careful with helmets.org, though, because if you read enough it might convince you that you should never get on a bike yourself, much less let your kid ride on or behind one.

We have a regular Chariot trailer and we started her around six months, no helmet, trails only, and she hated it until about eight months. We did not push it until she started to like it. Actually we took a break after our first attempts and waited until she was sitting very solidly upright on her own, and I think that helped. She is very small for her age so we use the infant insert for the Chariot.

She's a year old now and we've just started using a helmet (and riding on the road as well as trails). She loves it.

We have friends who put their kids in much younger than six months, and others who waited until a year. The one thing that has been pretty universal is that nobody uses a helmet until one year ... the trailers are very safe, but a helmet, ironically, is not safe for a baby's neck until about a year.

Some people also put a carseat in the trailer, but that isn't going to work with a Chariot unless the sidecar is much bigger than the trailer. There just is not enough room.