In Deathly Hallows, Harry visits Sirius's room and sees this:
The teenage Sirius had plastered the walls with so many posters and pictures that little of the walls' silvery-gray silk was visible. Harry could only assume that Sirius' parents had been unable to remove the Permanent Sticking Charm that kept them on the wall
I am assuming that Harry is right--there is a Permanent Sticking Charm on these pictures.
In OotP, Sirius says this in a conversation with Harry:
"You ran away from home?"
"When I was about sixteen," said Sirius. "I'd had enough."
So Sirius ran away from home when he was sixteen...but before that, he was able to put a Permanent Sticking Charm on his pictures, which is pretty complicated magic and would definitely violate that law about underage magic. Did this happen, or is something else going on?