Both assertions came as news to me. For about two hundred years after it arrived in the language in the thirteenth century girl was indeed a general term for any young person. But by the sixteenth century the word had shifted to our modern sense.
Youth is rather more complicated.
In one sense, it can be used for young persons of either sex, usually when we are thinking of young people as a group or in the abstract. We can, for example, speak of youth culture , a youth club , or a youth hostel . In all these cases, there is no distinction of sex. But these tend to be formal usages, as the examples show. All rights reserved.