remember there are some older chuunins like ibuki. even iff many of the chuunins look the same age it doesnt mean that they guaduated at the same time. there could be two 18 yr old chuunins right next to each other and ond could have passed 5 years ago when the other just graduated. also it may be possible that some people become chuunins by demonstrating skill outside of the test and granted the title by the village kage, but ... that would be a little unfair...seeing as how if you have the skill there is now reason that you shouldnt pass the test. your leadership skills only should get your team past the team events. but the exams are more or less just a sport to keep the countries from kicking each others asses and get new clients, so there are probally rules to circumvent gaining chuunin status with out taking the exam.