I would think so. What are you doing with the firearms, though? If you are selling them, then I would think you are definitely in the business of dealing in firearms, which requires a license. If you are obtaining the firearms for addition to your private collection for your own use, then it is no different than any other private FTF sale.
Firearms obtained in a private FTF sale must be only from residents of the same state, though. If you have any customers not a resident of Ohio doing the firearms trading, then you are violating Federal law.
Why the 21 age requirement? Is that a requirement for the tattoo work? It is not illegal to receive/buy a firearm, even a handgun, from someone 18 to 20 years old. In Ohio, however, it is next to impossible for an 18 to 20 year old to obtain the handgun legally, but not against Ohio law for them to sell or give away a handgun to someone 21 or over.