There's no single age at which all individuals gain the maturity to consent to sex.
True. But there are plenty of ages I, and any other normal person, can name where all individuals of that age cannot consent.
Indeed. And at those ages, there would be zero problem convincing a judge/jury/arbitrator/whatever that consent was impossible. As you just said, no normal person could possibly disagree. Hence, no number is needed.
Besides, supporting certain laws in our current State-dominated society does not make one any less libertarian. The State isn't going anywhere too soon. I support laws against murder. The violators of these laws are apprehended by police paid for with tax-dollars, prosecuted by people paid for with tax-dollars in courts presided over and staffed by people paid for with tax-dollars. If found guilty, they are send to prisons paid for with tax dollars guarded by people paid for with tax dollars.
That has nothing to do with the discussion. You said:
In particular, their leader, Ian Bernard, has some objectionable, and non-libertarian, views about the age of consent.
Even if we were to accept the argument that it's okay for a libertarian to believe that current age of consent laws are acceptable within the context of the present, imperfect society, that does not support the claim that someone who speaks out against that nonsense is un-libertarian. Arguing that there is some technicality under which your own views are tolerable does not equate with proving that someone who actually holds the correct views should be condemned.
And, of course, the argument that the current laws is tolerable, is not sensible. As I proved, the set ages could be stricken and replaced by simply making maturity an element of the crime, entirely within the current system. Therefore, no libertarian can support the current laws as some sort of "well, it's the best we can manage within the context of this Statist society," because it's not even close to the best that can be managed.
Anyway, you have made a
very serious charge against Ian, and you have entirely failed to support that charge.