The bifocal are the eyeglasses that have two distinct optical powers. It is most common used for correct both myopia and hyperopia. And the people who are over 40 need wear the bifocal glasses. At the same time, the bifocal lenses can reduce eye strain in early ages. However, it is enough for younger people to wear one single lenses of glasses.
As we know that bifocal glasses are made for people with two vision problems like presbyopia and myopia. They often occurs to older people who over 40 years. That because presbyopia mostly occurs to people over 40 years. That is to say, bifocal glasses are usually used for older people. Nevertheless, there are some young adult suffers such problems for lots of reasons. So, it is not strange to see a girl about 20 wearing bifocals.
Actually, we all know that bifocals can correct far-seeing vision and near-seeing vision. It Is mainly used to those who both have myopia and presbyopic, and there is obvious line between myopia lens and presbyopic lens. So, bifocals generally just help correct vision. But age is not the decision factors about myopia or presbyopia. If you must find a relationship between age and bifocals, now young people are easier to get myopia, and middle-aged, general above 40 years old people are likely to suffer from presbyopic. But there are also young people with presbyopic. So judging a man whether he would need to wearing bifocals from the age is not scientific.