I can only imagine the typical conversation between a defender of artificial age limits and a liberationist ending one way--with an enlightened ageist. Those who defend artificial age limits do so because their imagination hasn't evolved past 19th century assumption. As an experiment, if you just present them with this statement by educator and advocate John Holt, you're guaranteed to hear a particular nuanced set of responses and be able to test the limits of their reasoning.
"I propose...that the rights, privileges, duties of adult citizens be made available to any young person, of whatever age, who wants to make use of them." -John C. Holt, "Escape from Childhood"
They will remark that children shouldn't have the right to vote, for example, because they lack the mental capacity for it. This wouldn't be such a bad argument if they were referring to the potential ways adults could scam children out of their votes, but this is not how they mean it. They mean it in the most simplistic way possible--that children shouldn't vote because they don't understand politics. The same goes for sex--children shouldn't have sex because they don't understand sexuality.
The liberationist will chide back with the rebuttal: what about a 17 year old one minute before midnight on the day of their 18th birthday (the year a child can vote in the United States)? Are they incapable of understanding politics right up to midnight on their 18th birthday? What causes them to be suddenly blessed with the ability and knowledge to vote? The same argument gets even more confusing with age of consent laws, where there is not just one, but hundreds of different age limits, where suddenly children are capable of having sex in one area, but a mile away across the border, the very similar children there are incapable. What manner of science could explain this phenomenon?
It will cause the ageist to scratch their head, possibly never having concluded that there's a difference between 2 year old children and 17 year old children that the law might have missed. In any case, the ageist will retort that they understand the confusion over the exactness and absolute nature of the "one minute before midnight" scenario, but will usually respond by saying, "it's not an exact science, but there has to be an age limit somewhere even if it doesn't match when a child is capable of voting." One should note that this already flies in the face of their previous rationale.
The liberationist will chide in immediately with an obvious rebuttal. If what the ageist said is true, that there in fact needs to be a set age even if it doesn't correspond to ability (contrary to the reason the ageist had given in the first case), then why does the age have to be 18? Why not move the age of majority in the US up to 20? Or how about 30, or 40? If we understand that the age limit is arbitrary, but also understand that it must exist anyways, why do we settle on 18 definitively?
The ageist will then come back with how 18 is closer to when a person is capable of understanding politics than 40, and will then normally pretend they agree with the liberationist by recalling particular teenagers who are far more politically astute than their own adult colleagues. They usually do this to show that they are not bigoted about young people, but it does nothing but add to the case the liberationist is making. It's simply the point in the argument when the ageist has run out of explanations, just before settling on the "it is the way it is" rationale, unknowingly forfeiting all their earlier assumptions.
Age limitations have nothing to do with human aptitude. They do not legislate human aptitude. They are merely artificial limitations invented by humans to keep other humans from participating in areas of learning and experience they'd rather reserve for themselves. Lifting age limits on children would not impose the weight of the adult world onto them, and thus become its own form of oppression as critics may suggest. Children who do not have the means, the mind, the capability, the maturity, or the motivation, to make use of the rights newly bestowed on them, such as the right to vote, simply would not vote--just as adults do when they have neither the means, mind, capability, maturity, or motivation to vote. Society has already made it such that adults who are deemed incompetent for any reason are not permitted to bare the burdens and responsibilities that would come with competence--there is no reason to believe that the liberated five year old would be treated any differently than the incompetent adult when it comes to running their own affairs.
In fact, what society fears is not that incapable children would be dumped with the pressures and responsibilities that come with human rights beyond their ability, it's that in granting them such rights, many more than previously expected may be found more capable than the adult world ever imagined could be. Adults would be in a real state of cognitive dissonance over their assumptions. While five year olds would be sure to fail a required driving permit written test, for instance, we may just find that fifteen year olds, rather than just sixteen year olds, are competent enough to pass it. Such a thing would be sure to send shock waves of fear through the hearts of adults who would like to maintain the millennia-old belief that they, modern men and women, are special among all living things. It would force them to conclude that they are not special just because they've reach some magic age of human fellowship, and that they spent a good many years squandering their ability in pursuit of a magic number.