[MMM]

I think the issue is not only age, but more importantly context.

I don't know exactly how the question was asked, but if you said "Do you think it is odd to see non-Japanese wearing traditional Japanese clothing?" then it wouldn't surprise me that you had a high % of "yes" answers, because in most situations it is odd for ANYONE to be wearing traditional Japanese clothing.

For example, if you said "Do you think it would be odd for a non-Englishman to wear a suit of armor?", invariably, the answer would be "yes" from most people, British or otherwise.

However, if the question became "Do you think it would be odd for a non-Englishman to wear a suit of armor at a Renaissance Fair?" then I think you would see a dramatic shift in the answers.

I think the same is probably true if you asked "Do you think it is odd to see non-Japanese wearing traditional Japanese clothing at a summer festival?"... one of the only appropriate situations to wear yukata. You would find many more people saying "No, it's totally fine."