Custom becomes tradition when, as DWFII has rightly pointed out, it is passed on virtually unchanged from one generation to the next. It is the frequency of the observance of a custom, not necessarily its antiquity, that turns it into a tradition.

Generally frequency and antiquity are one in the same-- something between fifty and one hundred years. That said, something first done fifty years ago and not done again until today would hardly constitute a tradition. But something first done ten years ago, and repeated once a week, might be considered "traditional" today, although only the passage of more time could truly imbue it with the characteristics of a tradition.

Lasting traditions occur only within the mainstream of a society, not on the edges of that society. The "fringe", by its very nature, is in a constant state of change and therefore it is not capable of developing or sustaining customs that will evolve into traditions. Although the "fringe" does not develop traditions, it may, in some indirect way, contribute to the developing traditions of the mainstream society.

In Scotland, when Highland attire became broadly popular in the 1840s, the kilt was worn with the ordinary clothes-- shirts, jackets, shoes, etc.-- worn by everyday people. By the 1850s this customary mode of dress had been modified to be more comfortable and more becoming when worn with the kilt. From that point on this "style" of Highland dress was passed on, virtually unchanged from one generation to the next.

There is one thing that can be said about tradition, and that is that it is immune to radical change. There is no difference, other than minor points of style, in the mode of dress worn by a gentleman in 1909 or today in 2009.

Will traditional Highland attire change? Yes, but only in minor points of style. Tradition preserves those things which are good in mainstream society, and sloughs off the pointless fads that every decade produces.