ぼく is in principle for males who regards themselves as physically or mentally young and fresh. However, since 1960s also young girls have sometimes employed ぼく as the first person pronoun. At first the phenomenon began in schools for girls from upper class, and then it spread out into large communities. (I suspect the influence of
Hagio Moto and her colleagues, though unproven).
They cherished ぼく because it sounded to them sexually neutral. You know some children in
puberty cannot accept their given sex/gender so that they refuse to be treated in masculine/feminine ways. In general, such tendency is more conspicuous in girls because of, I think, the rapid physiological change. It also has some relation with the gender inequality still left in our society. Anyway, some girls began to use ぼく to express their refusal to accept feminine-self.
Today the use of ぼく by girls become so usual that you cannot think everyone with the pronoun has such sensitive thoughts. In fact it is just a fashion; there are plenty of girls using ぼく in animes and mangas, and most of them do so only to make themselves distinguishable from others. In other words, it is nothing but what is so-called 属性 (zokusei), a pretty tag sticking to a character.
In short, many girls now use ぼく which ever in reality or in fictions, but you should not regard it as regular / authentic / proper custom.
I opened the visitors board. You may write Japanese sentence there. I will correct it when I have time.