男生(nán shēng) - boy, male student, young man in Mandarin

男生 (nán shēng) started life as school vocabulary, the male half of a class, and it still lives there: 班上的男生, 男生宿舍, 男生比女生多. Its partner is 女生.

It has spread well past the classroom into ordinary speech about young men, roughly from secondary school to the mid twenties. Friends use it of each other, and 那个男生 is the natural way to point out a lad in a queue.

The trap is treating it as the general word for a male, the way English "guy" works. Call a man of forty 男生 and it sounds like a joke about him refusing to grow up. Once someone is properly adult you want 男人 for the plain noun, 先生 for the polite one, or 男的 in casual speech. Note too that 生 here is student, not born, so the sum of the two characters gives you nothing.