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.