"If I wrote a song 10 years ago and in my area it is traditionally sung at a certain time of the year or event does that song not become traditional?"

Of course - that nicely highlights the point that "written by someone" and "traditional" are not a pair of opposite meanings. The origin of a song is never traditional; tradition is what happens to it after it's been written, and it can happen during the author's lifetime.

"Trad" on a song sheet could be cynically described as meaning that the work of finding out who wrote it is judged to be more then the publisher's budget (including the possibility, in one extreme, that the information is not available at any cost, and in the other, that they can't be bothered to make the simplest enquiries)