The other night, we went to an okonomiyaki restaurant near the apartment, and after a while a group of 5 young guys came in and were seated next to my table. They placed their orders and a few minutes later one of the restaurant employees (a guy in his late 40's) came out to ask for their drivers licenses. The 5 customers were stunned. Amid all of the complaints and curses, there were statements of "it's ok, just bring us our drinks". The older guy succeeded in getting cards from two of the customers, but the other three refused to move. The employee explained that the restaurant had to protect itself and the other customers and couldn't sell alcohol to anyone whose age they couldn't confirm. Two of the guys could drink but the other three couldn't. When the employee left, all five were cursing, saying "I don't believe this" and "let's leave this crappy joint", and after throwing their tableware around they also left.
I did a search on "drinking age in Japan" and came up with a forum discussion on japan-guide.com that started back in 2003, with the latest post in 2008. Initially, the responses were that unless you looked young, you could buy alcohol without a problem, and at a minimum could get it from the vending machines. Over time, posters noticed that the number of vending machines selling beer had gone to zero, but that you still didn't have to worry about being carded. Seems that times have changed. The interesting thing is that the restaurant owner didn't cave in to the customers this time. Looks like the threat to lose a liquor license has gotten greater than the reality of having to lose customers.
(For reference, the minimum drinking age in Tokyo is 20.)