People's notions on this differ,but personally I think a 12-month old is generally not capable of really "getting" a trailer. In other words, I personally don't think it's something innate enough that you can teach a kid that can't understand what you're saying and is still figuring out a lot physically on their own. When they can do physical things like figure their way up/down a staircase, outside stairs, etc., then you can see they've developed an aptitude for knowing how to handle things like that. But even then, there's nothing to say they would know how to hold on to the bars of a trailer.
Generally, for most kids, you're going to be looking at 18 months to 2 years before they can handle a trailer.