Wednesday’s unseasonable warmth called for sunny day music. A happy coincidence then, that Ben Kweller, a purveyor of cheerful college rock, was headlining the 9:30 club that evening. Looking at the small crowd of underage early arrivals jostling for position, a question came to mind: is Kweller an artist or a phase? To paraphrase a famous film quote, it seems that he gets older but his fans stay the same age. The answer came…