An exception exists to the five year waiting period. The exception is directly written into the expungement statute at 18 Pa.C.S. sec. 9122(a)(3) as:

a person 21 years of age or older who has been convicted of a
violation of section 6308 (relating to purchase, consumption,
possession or transportation of liquor or malt or brewed beverages)
petitions the court of common pleas in the county where the conviction
occurred seeking expungement and the person has satisfied all terms and
conditions of the sentence imposed for the violation, including any
suspension of operating privileges imposed pursuant to section 6310.4
(relating to restriction of operating privileges). Upon review of the
petition, the court shall order the expungement of all criminal history
record information and all administrative records of the Department of
Transportation relating to said conviction.

This means that if you are convicted of a summary offense under 18 Pa.C.S. 6308, you are over 21 years old and you have satisfied all of the requirements from the initial sentence, your summary conviction shall be expunged without the five year waiting period. 18 Pa.C.S. 6308 involves summary offenses related to alcohol.

Therefore, your under aged drinking conviction can get expunged in less then five years.

In Japan its also illegal to purchase alcohol under age. However, you can buy beer from a vending a machine.