It doesn't sound very natural. I would say it this way (if I understood your intention correctly). - In some countries, (most) parents stop supporting their children financially once they reach eighteen. - In some countries, parents give financial support to their children only until the age of eighteen. * "leave one's responsibility" sounds a little unclear. "stop taking responsibility for ..." seems better. * "spend on their children" doesn't seem to convey the idea clearly. Taking full financial responsibility for someone can be phrased as "supporting them financially".

Hey Khaled, how are you?? Well, I'm not sure if I understood what you were trying to say in that sentence. Maybe it's not so clear, because you are using the verb to spend, and usually, we use a complement with this verb. So I can spend time, spend money, spend energy. So try to make this sentence a bit more clear, and we can work it out. You can also look at my teaching profile, I will be very please to solve this doubt and any other ones that you may have. Feel free to book a short class with me, so we can discuss this sentence a little bit more. Bye bye!