While I think it is impossible to put any sort of fixed date on the time line of Scottish history as to exactly when the kilt became "the National Dress of Scotland", I believe that the wide spread adoption of the kilt as the symbol of Scottish identity (as distinct from Highland identity) begins in the late 19th century/early 20th century. Certainly in 1914 Loudon MacQueen Douglas thought of the kilt in those terms when he published his wee book, "THE KILT A Manual Of Scottish National Dress".

Douglas was the President of the Scottish Society of Edinburgh (founded in 1911) and in the preface to his book he sets out the objectives of the Society:

"The cultivation of Scottish National Sentiment, the preservation of Scottish Traditions, and the encouragement of the wearing of Scottish National Dress."

So, in his eyes at least, the kilt was the national dress of Scotland. 100 years later, it would seem that this is so.