At that time being *20* and unmarried would be highly unusual, indicating some physical or mental defect. Judaism had and has individuals and groups with ascetic inclinations, but unmarried? Unlikely.
He most likely had brothers and sisters as well - since the Bible tells us as much (Mark 6:3) and would also be incredibly unlikely for a family to only have one child.
Paul remained unmarried. He was not regarded as unusual among the Pharisees, that is until he was converted.