君, さん, and ちゃん are the suffixes which is added to a person's name. 君 and さん are placed either after the given name or the surname. ちゃん is principally placed after the given name. 君 is used when you speak to your male junior in your office, to a fellow in your school, or to a little boy. さん is most commonly used. ちゃん is used as the diminutive suffix and rather to call a little girl or to show your affection or friendliness to a young girl. These uses are almost compulsory because Japanese people consider that calling a person without any of them are very impolite or abrupt. If you speak to your friend, your child or your younger brother or sister, you use none of them. If you speak to your elder brother, you call him お兄ちゃん or お兄さん instead of his own name and to your elder sister お姉ちゃん or お姉さん. If you speak your friend, your child or your younger brother or sister, you usually don't use none of them. --- Hope this helps

Actually, it's very complicated. It's not only for the age, but also for the social status/position, the relationship between us, depending on the person and family, etc. I'll write about "my cases". I would use さん(san) for: my bosses, the people whom I don't know, whom I meet for the first time, who aren't my close people the customers (san/sama), co-workers, some of my husband's relatives, my parents friends my friends who are older than me (however, I use "chan" for close friends of mine) the people who are older than me if I know their age/generation I use ちゃん(chan) for: small/young girls, female friends of my children, some of my female friends, some of my female relatives *except the male who has nickname with "chan" my children call almost all of my female friends with "chan" *it depends on the person/family some of my nieces/nephews call me with "chan" and some use "san" *it depends on their family I use くん(kun) for: small/young boys, male friends of my children, some of my male friends, some of my male relatives my children call some of their male relatives with "kun" and others with "san" After all, there are any specific rules. All are depends on the relationship between people. If you want to ask me more, please feel free to ask me. I hope this was helpful.

Mocchan gave an excellent and very detailed answer! 君=kun, さん=san, and ちゃん=chan. Senpai is for someone, who is on a bit higher grade than you, school-wise and university-wise.