It is difficult to square the notional unconstitutionality of this with the fact that the exercise of other Constitutional rights have long been conditional on age. This just looks like another example.

What is the consistent principle of law? I am having difficulty finding one that would support this ruling.

Laws limiting fundamental constitutional rights are subject to "strict scrutiny", which means they must be justified by a compelling government interest, narrowly tailored, and be the least restrictive means to achieve the interest in question. One might reasonably argue even that standard gives the government too much leeway when it comes to fundamental rights.

Age restrictions narrowly tailored to specific content thought to be harmful to minors have often been tolerated by the courts, but something broad like all book stores, all movie theaters, or all app stores violates all three strict scrutiny tests.

I'm interested: the only one that I can think of that has some limitations is the second amendment? Are there others?

As to the first amendment: Although not equal to that of adults, the U.S. Supreme Court has said that "minors are entitled to a significant measure of First Amendment protection." Only in relatively narrow and limited circumstances can the government restrict kids' rights when it comes to protected speech. (Erznoznik v. City of Jacksonville, 422 U.S. 205 (1975).)

The government doesn't have a compelling state interest in preventing you from downloading any app (a weather app, for instance) unless you provide your government ID first.

> In U.S. constitutional law, when a law infringes upon a fundamental constitutional right, the court may apply the strict scrutiny standard. Strict scrutiny holds the challenged law as presumptively invalid unless the government can demonstrate that the law or regulation is necessary to achieve a "compelling state interest". The government must also demonstrate that the law is "narrowly tailored" to achieve that compelling purpose, and that it uses the "least restrictive means" to achieve that purpose. Failure to meet this standard will result in striking the law as unconstitutional.

https://en.wikipedia.org/wiki/Strict_scrutiny

> It is difficult to square the notional unconstitutionality of this with the fact that the exercise of other Constitutional rights have long been conditional on age.

Some of this depends on whether the state has an interest in preventing known, broad harms - say in the case limiting minors ability to consume alcohol.

Conversely, there are no clearly proven, known targeted harms with respect of youth access to app stores (or even social media). What there are, are poorly represented / interpreted studies and a lot of media that is amplifying confused voices concerning these things.

> It is difficult to square the notional unconstitutionality of this with the fact that the exercise of other Constitutional rights have long been conditional on age. This just looks like another example.

> What is the consistent principle of law? I am having difficulty finding one that would support this ruling.

The Constitution of the US mentions age in a few very specific places, namely the minimum age to run for The House, The Senate, The Presidential seat, and I believe voting age.

I don't understand your point.

> the fact that the exercise of other Constitutional rights have long been conditional on age

Which of those are in regard to the 1st Amendment?

> This just looks like another example.

No, it doesn't.

> What is the consistent principle of law?

The 1st Amendment.

> I am having difficulty finding one that would support this ruling.

The judge stated it clearly. And if there's an inconsistency then it's other rulings that violate the 1st Amendment that aren't supported, not this one.