Legally speaking, it is. A paternity suit is a legal issue, and all that would matter with regard to any such suit would be guilt or innocence in the eyes of the law. But that does not even appear to matter. If a convicted male rapist can sue for visitation rights, then a convicted female rapist can sue for child support. Of course, in this case there was no rape conviction, there was not even a charge of rape. The guy had a several year window of opportunity to make his case, and he did not do it. No one else should be to blame for his inaction.
No, a person that is guilty (legally speaking) is not therefore guilty. An innocent person that has committed no crime yet subsequently convicted is guilty (legally speaking), but we ought not therefore conclude that such a person is guilty. The lexical definition stands in strong contrast to the stipulative (hence, legal) definition.
I won't say the 20 year old is guilty, but I will say the 20 year old is guilty, where the former is legally speaking and the latter is not. Clearly, you're using the term as anyone in the legal field would, but if it's clear that she is guilty; hence, if it's clear that she committed the crime, then not being found guilty one way or the other because no charges were ever brought up has only a bearing on guilt in the legal sense--not the lexical sense.
Yes, maybe the legal sense is the only sense that matters to the courts in regards to legal issues, but we shouldn't let the convoluted different uses of words cloud our moral judgement. Outrage shouldn't be a function of a legal verdict--or lack thereof. Recall from what I said earlier, if a person has committed a crime, then that person is guilty of a crime. No differing legal usage is going to change that fact. We can presume innocence all we want.