He was only 12 when He disobeyed his parents, staying behind in Jerusalem to teach in the temple after they had started home.
He didn't disobey His parents by staying behind. They thought He was with relatives, in the caravan. The fact that they frantically headed back to Jerusalem when they discovered that He was not with them, and then searched for Him for three days, shows that they had no clue where He was. He probably was totally surprised that they were so worried about Him, and that they didn't know what He was doing. He was a 12 year old boy, wanting to be independent and to get on with His work, and yet still devoted to His parents.
It’s not a trivial matter. The guy literally and blatantly accused Jesus of breaking the law - sinning. It’s as bad as disputing the Virgin Birth.