Marco Rubio has been hailed as the front-runner for the Republican nomination and yet, for all of his promise, his support is around 12 percent in the polls.

One possibility is that he is broadly appealing at the expense of being anyone’s first choice. From this point of view, the problem is that he hasn’t yet made a winning pitch to any one wing in today’s deeply factionalized Republican Party, and he doesn’t have the resources or elite support necessary to muscle his way to the top.

But another possibility is that voters have been slow to flock toward Mr. Rubio, who is 44, because he doesn’t quite look the part. For some, his youthful charisma might border on un-presidentially boyish.

Some New York Times readers are sympathetic to the possibility. Last week, my colleague Toni Monkovic culled comments from readers showing that many thought he was too young. I asked my followers on Twitter whether they thought appearing to be a little too young hurt Mr. Rubio, and 41 percent of the 356 deeply unrepresentative respondents said “yes.”