Hi, My name is Philip. I am an attorney with over 16 years experience. Hopefully I can help you with your legal question.
You have the correct code citation
ORC 4301.69 is the law that would apply.
If you take a look at the statute (you can see it here http://codes.ohio.gov/orc/4301.69
the law does NOT require the defendant to be over the age of 21.
Specifically the statute provides
Except as otherwise provided in this chapter, no person shall sell beer or intoxicating liquor to an underage person, shall buy beer or intoxicating liquor for an underage person, or shall furnish it to an underage person, unless given by a physician in the regular line of the physician's practice or given for established religious purposes or unless the underage person is supervised by a parent, spouse who is not an underage person, or legal guardian.
So if a person provides (furnish) alcohol to a person under the age of 21 they violate the statue.
So it could be that a 18 year old furnishes alcohol to a 20 year old...that would violate the statute and would be a separate crime from "minor in possession"
Please let me know if you have more questions...happy to assist if I can