When we use a "NOUN + THING" combination, we are saying that the topic at hand has to do with that noun. It is connected to that noun. It is related to that noun. In the comic strip one shark comments that his going in circles and getting nauseous must be related to/ have to do with his age. It's a very common and easy way to connect ideas.
1. When women do not want to explain what they're talking about or joking about, they will say to guys, "It's a girl thing". It may be private, personal, embarrassing or difficult to explain. When a woman says that, do you get curious… or do you lose interest?
2. Likewise, when men don't have the patience to explain why they like watching football, going fishing or having a beer with their friends… They say "It's a guy thing". Are there certain things that you don't have the time or patience to explain because you think people won't understand anyway?
3. When a conversation is interrupted by a telephone call, once the person is finished chatting, if the call was from a colleague at work and was very technical, normally we explain to the other people, "It's a work thing". That way no one will think it is interesting and will probably lose their curiosity. Do you say something similar in your own language?
4. Anything we find difficult to relate to or explain, we use the "noun + thing" construct. For example, "It's a teenage thing." Are you totally out of touch with teenagers these days… or do you understand the trends and issues they are facing?
5. Often parents say to the children, "Don't worry… It's an adult thing." Do you now understand all the things your parents wouldn't explain to you when you were a child? Would those things have been difficult to understand when you were younger?