When does someone stop being a child?
Ephesians 6:1-2. "Children" has no absolute age-associated concept to it, does it? If it was only used in the sense of young children, well then. But it refers all over scripture to offspring generally. When does the principle of a child as a minor actually come into play here?
Children (not adults) are to obey parents "in the Lord" (Ephesians 6:1-2). If a parent's demand conflicts with righteousness, then the Lord is obeyed over the demands of a parent. The word for “children” is tekna. It is the same word used in Matthew 18:1-6, which clearly indicates a young human. It is sometimes applied to adults because we are children of God (spiritually, we are not fully mature). It can at times refer to descendants, such as in I Timothy 5:4.
Notice in the same context that Ephesians 6:4 tells fathers to bring up their children. The Greek word ektrepho means to “bring up from childhood.” At some point, a child is no longer a child and has been raised (past tense). Thus, children (young humans) are to obey their parents. And people (regardless of age) are to honor their parents. Notice that Ephesians 6:2 doesn't use the word "children."