Here in North America, the original release of Miitopia on the 3DS snagged an E for Everyone rating from the ESRB. While the Switch version of the game hasn't been rated yet, it'll likely secure the same rating, as it's mostly a port of the original. Over in Russia, the game's age rating is very different.
While the 18+ rating is usually reserved for titles like Grand Theft Auto, Miitopia's Switch release in Russia will indeed be rated 18+. This is due to the game's portrayal of same-sex relationships, as they go against the country's bill "for the Purpose of Protecting Children from Information Advocating for a Denial of Traditional Family Values." Any content with same-sex relationships is automatically slapped with the 18+ rating in Russia with absolutely no exceptions.