They sit on top of Democratic presidential primary polls, but as Nate Silver notes, those numbers may not yet reflect the age issue that will be an obstacle for both former Vice President Joe Biden and Sen. Bernie Sanders, I-Vt. Though both men will likely have to confront the issue at some point, it will likely be a bigger challenge for Biden to overcome.

At the outset, it’s worth making clear that age should be considered a legitimate issue for both candidates. If elected, Biden would be 78 on Inauguration Day, and Sanders would be 79. By way of comparison, Ronald Reagan, the oldest president ever to serve, was still weeks away from reaching 78 when he left office. When Reagan made the famous quip in a 1984 debate about opponent Walter Mondale’s “youth and inexperience,” he was still only 73.