It's not that they don't think a teenager would break the law, it's that it's a convenient lie that everyone participates in so that they can have beer vending machines.
It's the same with gambling. Gambling in Japan is mostly illegal, but also extremely popular. Instead of winning money, you win prizes, which due to the way the law is worded is not technically gambling. The prizes are standardized, and next door to the pachinko parlour there is another, totally unrelated shop that exchanges them for a fixed cash payout, and then sells them back to the place you were gambling^W winning prizes at.
Same with whale meat. It's all caught for "scientific purposes". They have to regularly check that it's still delicious, as part of their ecological efforts to monitor the marine environment.
Everybody understands what is actually happening, they just don't want to get into a discussion about it.