The grandson of the revered songwriter Terry Allen, the 23-year-old is bridging generations of Lone Star state artists — and playing Rolling Stone's upcoming Stateside festival Despite being born into a family of Texas songwriting royalty, it took Calder Allen a while to come into his own. He may have been raised in Austin, the grandson of musicians Terry and Jo Harvey Allen, but Calder’s original goal was to simply become a fly fisherman. He even had a chance to work for one of his mentors, J.T. Van Zandt — yes, the son of Townes — as a guide on the Texas coast. But just as Allen was getting his feet wet in the guiding business, his family history grabbed him. “I knew that I had to give music a fair chance,” Allen says. “To this day, it’s still one of the hardest no’s that I’ve ever had to say.” But Allen’s decision is proving to be a fruitful one.…