Wanna feel old? Watch Euphoria. HBO’s arty, thoroughly depressing teen series returns for its second season on January 9, after a nearly three-year wait. One feels old watching the show because, yes, it’s all about young people and their Gen Z hangups and habits. But there is also, on the happier side of feeling gray, a mounting realization that much of what’s being shown on creator Sam Levinson’s series is the petty, impermanent stuff of youth. The pain may be real, and may be turgidly articulated. But it will, in large part, pass.  For some of the characters, anyway. Euphoria’s second season splits itself in two. On the one side…