Is it illegal for someone under 16 and someone over 18 to kiss?

Arhh well in some jurisdiction's yes, because the girl underage did consent to kissing.. This however totally depends where your from.

In Australia you can get charged because the age of consent is not only limited to intercouse but a whole range of sexual activities even when you give the girl some dirty look and she take's it sexually. you could be charged with sexual assault. Weird but yeah the reason why is this the minor is still considered to have little understanding of whats right. Even peso's get arrested for kissing younger girls it happened b4 without intercourse