The problem with all the discussion about ball handling is that nobody wants it called the same way.

Half the coaches I talk to want the refs to "let 'em play" at the younger age groups. After all, volleyball is supposed to be fun, many of these girls are in just their first or second year of playing, and younger girls don't always have the upper body strength to set well. In short, calling doubles discourages players from using the skill they've been practicing.

Half the coaches I talk to want the refs to call the doubles at younger age groups. After all, they'll be expected to execute the skills cleanly as they get older, not calling doubles on a poor setter provides an unfair advantage to the lesser-skilled team, and the players don't work hard to fix their setting mechanics unless they realize they'll be called during a match. In short, calling doubles encourages players to practice the skills they'll be using.

All I know is that it irks me to no end when I see referees call 13 year olds for double hits that they would never, ever call in college, and I'm not even talking about the "new, even more relaxed" approach.

Chances are good those officials either don't work many college matches, or they have adopted the second philosophy above.

A lot of USAV referees simply do not seem to understand that a ball can easily spin off of two hands without it being a double contact.

Apparently, then, even less coaches understand that. I'll quit calling the sets that spin wildly when the coaches quit screaming at me for not calling it. I'm very aware that a ball can be made to spin in a number of different ways without it being a double, but for some reason when it involves an overhand setting motion, there's an expectation by coaches that "if the spin's bad enough, it must have been a double" - even if that spin is in the forward direction. This may not be from every coach, but most coaches (regardless of the ruleset or level of play) feel cheated when the other team sets a ball with much spin and it isn't called.

But in any case, why should calling double hits get more relaxed as players get older (and presumably, more skilled)?

Calls shouldn't change (too much) by age. One problem is that each venue (high school, club, and college) places a different expectation on what is called a double. The change in college rules is intended to narrow that gap somewhat, bringing women's college ball more in line with FIVB (and, by extension, USAV).

I would say that it appears to get looser as players get older because it's being called too tight at the younger ages (most specifically, high school, but sometimes at the younger club age groups as well, as noted in your first comment).