It's not really an age limitation, but it's much easier to use "chan" for kids. Basically -chan for girls and -kun for boys is used pretty much automatically f

It's not really an age limitation, but it's much easier to use "chan" for kids. Basically -chan for girls and -kun for boys is used pretty much automatically for smaller kids, even if you meet them for the first time. For adults it's more complex, it depends how close you are, how that person is used to be called among family/close friends, etc.