Interesting question. As far as how old," in my view it would be the age where the child can support his/or her head while wearing a helmet. And the rub is, helmets dont fit infants younger than one year. So thats your cut-off point. When your son is one, you can put him in a trailer. I would absolutely not advocate placing a child carrier in some sort of trailer and trying to make that work. Youve really seen people try that?
In short, for a while youll need to enlarge your carbon footprint to keep your son safe.
There arent a zillion convertible" trailers out there, but some do exist. Maybe the best is the Baby Jogger Switchback stroller/trailer ($579; babyjogger.com), which converts easily from stroller to trailer without a conversion kit or tools. And the front wheel thats used in stroller mode fits neatly into an on-board pocket. It also will seat two. Burleys DLite ($460; burley.com) is seen by many as the best child bike trailer on the market. It converts to a stroller with the purchase of a $60 kit. Or, Schwinns Joyrider ($380; schwinnbike.com) offers trailer/stroller functionality at a bit lower price. Not quite as spiffy as the Baby Jogger or Burley units, but a solid trailer.
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