it is like you have one of those wooden gates

It's not, though. It's not a random wooden gate in the middle of some unguarded park. It's a TEMPLE that is still in use by Buddhist monks, inside world heritage site. A 17 year old kid knows to gauge that difference.

It was at Toshodaiji, which is a 1200 year old very well maintained temple and a designated UNESCO world heritage site. I am very doubtful that he could be unaware of what the temple is, as its very much a tourist attraction and not just some random buildings. He is a kid, so I dont think it merits anything extreme in terms of punishment but the crime in this case is pretty serious. But he is 17 and definitely knew better. Any adult would likely face legal consequences.

It's not exactly rocket science to avoid vandalizing an active temple.

There are other things literally everywhere that the 17 year old could have damaged instead.