I am sorry that I took so long to write this post, but I realize that this is a complex issue, and so I would like to attempt a thoughtful response.
First of all, on activist judiciaries:
There are three definitions of 'activist judiciary' I've seen people use: One, as an insult against any judges they disagree with. That's silly and I'm just ignoring it for our purposes. The second one is a judiciary that does not exercise restraint and respect for the status quo, but rather rules based on what the law says as far as they understand it. The third is the idea, expressed by some Progressive judges (most famously Oliver Wendell Holmes) that it is the role of the courts to interpret the law to bring about a more progressive society, even when the interpretation is patently not in bearing with the original text. This is also the 'living constitution' approach.
We already know that if the progressives get into the court, they will twist the Constitution as much as they want to to accomplish what they think is 'the right thing'. The answer is not to have a court that defers to the status quo. The court must defer not to the status quo, nor to our notions of 'utility', but to the law.
Second, on the issue of RKBA and pre-18-year olds:
I will assume that you support a strict scrutiny standard on the interpretation of the 2nd Amendment, the same standard utilized by the courts to free speech cases. If the Washington State legislature enacted a law prohibiting anybody younger than 18 from writing a letter to a newspaper, posting on the Internet, or otherwise expressing their political opinion without parental consent, you would no doubt be outraged, and properly so. Nowhere in the Constitution can you find the backing for such a thing.
Now, consider the strict scrutiny standard. Under this standard, a law must satisfy a compelling government interest, be narrowly tailored to achieve it, and must use the least restrictive means possible to achieve that end. If all three of these are not satisfied, then the law does not meet this standard.
Now, what we want to do, I presume, is... prevent youngsters getting into gangs? Ten-year-olds buying cheapo pistols with their allowance and shooting each other with them? At any rate, an overall ban on firearms ownership for everybody younger than 18 years old is neither narrowly tailored for this purpose, nor is it the least restrictive means.
What is worse for the law's authors – as the dissenting judge clearly pointed out – is that a 17-year-old has not been viewed as a child during the Framers' era. Males of this age were part of the militia – are still, legally, part of the militia, and young men as young as 16 years old were accepted into the Continental Army. One of these seventeen-year-olds was Lafayette – can you imagine Lafayette being arrested due to not having parental consent to pack his pistol and sabre?
I understand, of course, that it is not realistic to expect the court to throw such a law out in the current political climate. But I don't see any reason to pretend this law is just, fair, or compatible with strict scrutiny.