The characters in Dohna Dohna are factually at least 18 years old, and therefore not minors. This is clearly indicated on the website of its original English storefront, Johren.games:[1]
Dohna Dohna is also now sold on the following download stores as of August 8, 2023, which also clearly state that the characters depicted in sexually explicit conduct are at least 18 years of age:
JAST USA (jastusa.com),
Based in California, United States, which states on the product page: