The word plebis is simply the genitive singular (there is no plural) of the collective noun plebs. In this particular instance plebs urbana is more or less synonymous with the portion of city dwellers not wealthy enough to be included among the equites and not included in the senate or senatorial magistracies. There is nothing in the word plebs itself that precludes the inclusion of individuals of both sexes and any age--the plebs urbana were, simply put, just the people of the city, and there's nothing more implied by the term itself. In this case, however, Augustus probably means the heads of households. Distributions of coin and grain to the plebs were commonplace political tactics in the Roman Republic, and became the sole privilege of the emperors during the Principate, and it is somewhat unclear precisely how the individuals that received these distributions were counted. It seems most likely, however, that only the heads of families were included in the count, who were then supposed to share what they were given with their families--mind you, this also means that the idea of the Roman plebs living on the grain distributions is patently absurd, since the distributions were only barely enough to feed an individual and totally insufficient for a family.