Hi Lisa, I'm not sure about a bike trailer but Kellie said her pedi told her 15 months for a bike baby seat. I suppose as long as they can sit up and you strap them in good so they don't go bumping all over it would be okay. I could be wrong though..
I haven't really checked them out yet, but they go both ways. Cheap and expensive. My sister has a cheap one and it shows. I peeked inside and the bottom was like a hammock. they just sat on this piece of fabric with hardly any support and I don't think that is good. THere's no, like "shock" cushion you know. When I take Blake in the jogging stroller I'm amazed at how much is absorbed and how much he really loves it. Let me know if you find a good one. I think they should be seat belted in and around 15mo's? Maybe ask the Pedi?
I wonder if you posted this on the Baby Products forum if it would get more responses... I'm curious about what people think on bike trailers too. We were looking on Craig's list this week and the price ranges are pretty broad!
9 months with the Shwinn brand from Walmart. He loves it!
As long as they are strong enough to sit unsupported t should be OK. While there is a harness, it's not as secure as a car seat harness so they would need trunk control to keep themselves upright. You don't want to have to keep stopping and undo all the coverings, etc...just to sit them back up. LOL! :)