Against copyright, for age of consent.
In general libertarianism is voluntarism taken to the extreme, with no "general good" and emotion allowed to interfere. So common arguments for all variants are such:
Libertarian arguments for copyright are based on you accepting the agreement while buying or receiving something. If you don't, then somebody has done that before you and violated it. Fruits of a poisoned tree.
Libertarian arguments against copyright are based on you and the authors having no other option but to use what's given with such an agreement, and with you being deceived while told you are buying it (which would mean you can copy all you want), and in case of any technology patents with laying claim on a resource which isn't depleted by sharing.
Libertarian arguments for age of consent are that children are not conscious enough to consent. That part is common, then variations follow. For some it makes them property of their parents, who can decide anything for them, but if after becoming adults they consider it a violation, they will be in their right to treat it as such. For some - without that "but". For others it means that some axioms need to be chosen so that parents could, well, feed and teach and discipline their children, but couldn't abuse them. For others it's going to be managed by a community which will ostracize parents mistreating their children.
Libertarian arguments against age of consent are obvious - they are alive so they can consent.