At some point in the last twenty years or so. I can perceive the gender of the term. However, my wife is much younger than I, and she and her female friends exclusively use the term "guys" to refer to themselves and cannot comprehend that I can perceive gender in the term. I mean, intellectually, they know it use to connotate gender, but it is silly to them in the same way that it's silly to us that strawberries are berries that grow in straw. In the US, and apparently also depending on your age, "guys" is gender neutral. And as a hobbyist linguist, "we do not prescribe language, we describe it"...