This is of course a perennial question, and part of the broader debate about when, if ever, strict liability in criminal cases (under which a defendant is liable without any showing of a culpable mental state) should be allowed; and it is exacerbated when the conduct that the defendant reasonably thought he was engaging in (e.g., having consensual sex with an adult) is constitutionally protected, after Lawrence v. Texas. When I last checked, strict liability was still a majority view, though a substantial minority of states (including my own California) treated reasonable mistake of age as a valid defense, and a few of them even concluded that strict liability was unconstitutional, at least as to serious crimes.
The Texas Court of Criminal Appeals just handed down several interesting opinions on the subject. Six judges concluded Texas law provides for strict liability, and there’s no constitutional problem with that; one of the six elaborated further. One other judge among the six thought Texas law, properly read, doesn’t provide for strict liability, but reluctantly joined the majority on the grounds that prior precedent had accepted strict liability. Three other judges dissented, concluding that strict liability was improper and even unconstitutional in such cases. Much worth reading, if you’re interested in law and criminal culpability.
For my thought on the Lawrence question, see this post (which is relevant to the general issue, though not necessarily to cases where the defendant knew the other person was under 18 but claims he reasonably believed that she was above some other statutorily indicated aggravating age, such as 14).