What`s your opinion on underage drinking?

Under these conditions: They are in their late teens (17-18) They always have a DD. They drink once or twice a month (3 at the absolute most), and drink in moderation (enough to get buzzed, but still standing and walking normally and making responsible decisions) like about 4-5 beers or a couple shots. Is it really a problem then? And would there be any significant health consequences as long as they stuck to 1-2 times a month?