Declaring ownerless sabbatical year fruits

When the time came to destroy the produce of the Sabbatical year, I declared the produce that I had ownerless in front of three people. Afterwards I noticed that one of these people, although he’s past his bar-mitzvah age, is unmarried and lives with his parents. I wasn’t sure if a person who lives at his father’s expense is considered as having reached majority in this context.

There is no problem whatsoever. Even someone who lives at his father’s expense is considered to be an adult, and you can declare things ownerless in front of him.