HP, SG1, and "Canon and the Greater Reality" on Sunday morning

HP Question
Do we know when wizards are considered by the MoM to be "of age"? I was thinking today about how the kids aren't to use magic in the holidays. But in GoF, we learn that Fred and George have spent the summer inventing magical gag items in their room. We know they're 16, since they're too young to participate in the tournament.

So, do you think 16 is the age of majority? Or does the Ministry not track the magic that carefully and so children in homes where adult wizards are using magic can more easily get away with breaking the Decree for the Reasonable Restriction of Underage Sorcery? Or is what they are doing somehow not quite sorcery?

ETA: Looking at the letters that Harry receives from the MoM, they both say We have received intelligence that..., which seems to indicate that they don't actually know what is happening. And in fact, it was Dobby, not Harry, who used the Hover Charm, so they can't be tracking Harry's wand. Thusly, I conclude that Fred and George are probably safe concocting things in their room so long as Percy doesn't notice what they're doing.

Stargate Question
I've been tossing around (and tried to write at least once) some thoughts about Daniel and Paul Davis during 48 Hours. One thing I'm trying to work out: do you think Paul speaks Russian? I'm pretty sure we already knew that Daniel does. I realise that we saw all their negotiations in English, but that's because this is NA TV.

General Thoughts about Character Extrapolation and Interpolation
This brings up a question I've often wondered about (and possibly have posted about before): when the canon is restricted by its medium, do we restrict our interpretations in the same way? In the Stargate example above, the characters all speak English in the Russian negotiation scene because it's for an English audience. But does that mean that when I'm writing fic about it, I should assume that that's always the case?

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