PA, like many other states, applies a "social host law" to people who decide to serve alcohol at a private party. If you sereve alchol to a minor and they get hurt the host can be sued.
If you are caught buying, drinking, possessing, or transporting alcohol before age 21, you can be arrested. Under PA 0 tolerance law, people under 21 can be arrested and charged with a DUI even if their blood alchol is under .20
If you are convicted of under aged drinking, you can be fined up to $300 , jailed up to 90 days , or both. Also, you can have a criminal record and loose your drivers licence for up to 90 days.
If you are caught under D.U.I. (Drinking under the Influence of Alcohol) the maximum penalty is up to $5,000 fine and up to 2 years in jail.
If you are tried and convicted, you must pay a minimum of $300 and spend atleast 48 hours in jail, and pay other legal fees.