Old songs by unknown authors are not necessarily traditional: they are just old - many of the Child Ballads existed only in written form 100 years ago and have never been collected from "source singers"

Being "traditional" is not a fixed attribute - songs are entering (and leaving) the tradition all the time: I reckon the average lifetime of a song in the the people's consciousness is three generations. Anything older than that which is still sung in non-professional settings can be reckoned to be traditional, irrespective of its origin or authorship. The songs in my tradition are what I heard my parents and grandparents sing plus other newer songs I learned while growing up.

For example, songs of WWII were well known to my grandparents (the WWI generation) and my parents (who fought in WWII) and were part of their tradition. I learned many of these from them, but my children know far fewer of these than I, and their kids will know hardly any. The rest will not be passed on, and will gradually fade from the national consciousness, to be replaced by succeeding generations of songs.

I learned loads of contemporary songs growing up in the 50s and 60s ranging from Dylan, Seeger etc to "The Wild Rover" and "How Much Is That Doggy In The Window". My kids learned many of these from me and their kids will also learn from me and them.

I guess that probably less than 1% of any era's songs will pass to the next generation and 1% of those to the next and so on. What is left is the enduring tradition. Songs like Doggy In The Window, Delilah, Simply The Best and many Beatles songs are now well on their way to becoming traditional songs: more people know them than know where they came from. We could find out who wrote them if we really wanted to but it doesn't matter much to us; we may have a fair idea of who recorded them, but that's not really important either, because traditional songs are the songs that we sing.

Some may have been absorbed with mother's milk, while others have been provided by Tin Pan Alley - the folk taste is all encompassing. Collectors may have deprecated vast swathes of their sources' repertoires, but traditional singers from Joseph Taylor through Phil Tanner to Walter Pardon sang "unsuitable" and "worthless" commercial minstrel and music hall songs as well as the "old songs".

A good song is a good song whether 500 years or 5 minutes old, whether the author is known or not. And good singers love good songs, and will take them from wherever they can find them. The best will survive to become the next generation of the tradition: when they no longer speak to the people they will be forgotten and replaced by more relevant songs, new or revived.