In contrast to different Canadian cigarette package warnings, each cigarette package sold in Japan comes with the same pair of warnings:
未成年者の喫煙は禁じられています。
Smoking by minors is illegal.

あなたの健康を損なうおそれがありますので、吸いすぎに注意しましょう。
Excessive smoking can degrade your health, so be cautious of it.

Translation mine.

That's all. The warnings could be in small black letters on dark blue background, for all they care.

Smoking by Minors is Illegal.

It is illegal for a person under 20 to consume (smoke, chew, etc.) tobacco. The penalty for this is a ¥0 fine (reduced since original amount) for the parents (or a legal guardian) to pay, and confiscation of all tobacco products and paraphernalias posessed by the minor. It is illegal for a person to sell tobacco products or paraphernalia to minors if aware of the minor's intent to consume tobacco. The penalty for this is a fine of ¥500,000 (aprox. $5,000).

Consequently, it is completely legal to sell cigarettes to innocent 4 year olds in Japan. Cigarette vending machines operate legally on the streets without ID checks.

Excessive Smoking can Degrade Your Health, so be Cautious.

Seventy-five percent of cigarettes sold in Japan comes from Japan Tobacco (JT). JT is the world's third largest tobacco company (Following Philip Morris, British American Tobacco), and is owned two thirds by the Japanese government in shares. Cigarettes can be purchased from vending machines within Japan's Ministry of Health, Labour and Welfare buildings, at low tax rates (after tax, a cigarette pack costs around ¥250 ~ ¥280 (around $2.00 ~ $2.30) ), with inconspicuous health warning labels.