Young planets (<1Gyr) are helpful for studying the physical processes occurring at the early stage of planet evolution. TOI-251 b is a recently discovered sub-Neptune orbiting a young G dwarf, which has an imprecise age estimation of 40-320Myr. We select TOI-251 sibling candidates based on kinematics and spatial proximity to TOI-251 and further use the color-magnitude diagram to refine the list and to compare to multiple open clusters. We report the stellar rotational period for 321 sibling candidates in a 50pc radius around TOI-251 by analyzing their stellar light curves and find a color-rotational period sequence that lies in between the Group X (300Myr) and Pleiades (120Myr) members, suggesting an age ~200Myr. A quantitative age analysis using gyrochronology relations gives 204+/-45Myr, consistent with the average Li age of selected siblings (238+/-38Myr) and the Gaia variability age (193_-54_^102^Myr). The detection fraction of comoving candidates that have a short rotational period is 68.1%, much higher than the typical value in the field (14%-16% from Kepler). The overdensity of young stars and consistency in age of stellar siblings suggest a potential young association candidate in the Phoenix-Grus constellation. Though TOI-251 b has a radius larger than most of its field-age counterparts, we are uncertain whether TOI-251 is inflated, due to a lack of knowledge on the planet's mass.