Underage magic?

[Judy]

Sharon said:
> As for Morfin and the Riddles; the Ministry detected the magic done 
> to kill the Riddles.  It did not know who had done the act because
it 
> can only detect magic, not the person who performs it.

Oh, very good point -- the Riddle murders really aren't related to
the problem of magic detection by the MoM.  Still, there is a general 
problem with the MoM and its ability to detect magic being done. 
Harry was detected casting a Patronus but various members of the Order 
weren't detected using magic when they went to retrieve him a few
weeks later; nor was Dumbledore detected when he did magic at Privet 
Drive at the beginning of Book 6. Plus, the various magic acts that Tom 
Riddle did on Morfin (stunning, memory modification) weren't detected, 
either.  It doesn't seem that the MoM detects *all* magic.

-- Judy