Calling someone a minor can be seen as offensive, or just plain rude. It can make you seem like a jerk or a douche if you're calling someone a "minor" if your conversation has nothing to do with the law. In a way, it could be verybelittling to them.
In most countries, any person(s) who is under the age of 18 years. (See example one.)
Less important or urgent (in comparison to other situations or events); has less priority than something else, or something that is not too difficult to solve or clear. (See example two.)