I remember being shocked how many kids were totally disinterested and disengaged from the thing they were dedicating close to half of their waking hours. That never happened when I was homeschooled - if I wasn't interested in something, or didn't like how something was being taught, it could be done differently. I understand that's a very labor intensive way to teach, and I don't blame kids for being disengaged from the unpaid fulltime job they're being actively coherced into doing. I want to believe school shouldn't be mandatory, and that a lot of kids would benefit from dropping out and getting an apprenticeship in a field that interests them, traveling, or doing ANYTHING that actual engages them and gets them to develop some kind of expertise (even playing videogames or smoking weed all day can built a knowledge base that's useful for many careers). 

In my case, the lack of structure in unschooling hadn't exposed me to enough academic community to inspire me. Sure, we went on group trips to museums and did experiments, but that wasn't the same as having an actual program with a clear path to a field.  At 14 I wanted to be a chef or a massage therapist. After starting school, and getting into AP STEM classes with other motivated kids, I realized I actually do really enjoy math and science and ended up majoring in physics in college. 

I also think a lot of parents underestimate the amount of work and expertise that's required to homeschool, and that can strain the relationship kids have with their parents. For very independent, free-thinking kids who love to read it can work very well. For an anxious, distractible kid like me who needs a little more structure it was a frequent source of conflict. Schools can also act as a site for social workers to provide support in a way that's very difficult to emulate at home. Parents often miss psychological disorders like anxiety, depression, and ADHD in their kids that teachers can recognize right away (I'm having trouble finding the source I had for this, can anyone support or contradict this in an empirical way?) and I suspect I would have had access to medical interventions for some minor mental health problems much earlier on if I had been in school. 

They both had pretty negative experiences in grade school, and felt that school was more about "warehousing" and "preparing kids to be cogs in the capitalist machine" than actually teaching and nurturing kids. I agree with that to an extent. 

I'm an only child, which may have exacerbated some of the problems I've mentioned in other comments. Growing up in the Bay Area, there was a pretty active community of unschoolers, and we'd meet up at "Park Days" once or twice a week. Unfortunately, the homeschooler community can be pretty fragmented based on which philosophy of homeschooling you follow. I remember there being many instances of clique-y-ness among the parents (less so among the children somehow), cases of parents not talking to other parents, and groups splintering off into other groups. The result was that the group my family was "in" with was pretty spread out. I had a community, most of them just lived 1hr+ away. 

There were a lot of opportunities to dive deep when cool things presented themselves. For example, each year I got to go with my grandma to a natural products convention for two weeks and learned a lot about the supplement/vitamin industry. When I got Rosetta Stone I got to drop all other subjects and binge learn Spanish for a month. That definitely wouldn't have happened in a traditional school.

I think there's potential for it to go well/poorly at any age, and I'd encourage shifting the focus on the specific way the kid can be unschooled, whether they want to or not, and how well suited to it they are.  A lot of parents in my community saw their decision to unschool as protecting the early childhoods of their kids from the stresses of a culture of "academic rigor" that they saw as stress inducing, mind-numbing, and turning kids into cogs of a capitalist machine, and then later allowed or encouraged their kids to go to more traditional or independent high schools. Other parents had kids who did well in traditional elementary school, and were then severely bullied once their peer group hit puberty, to the point their parents looked into radical alternatives.