Democrats should probably start thinking about grooming a fresh-faced bench for 2020. 

The 2016 election just ended, and President-elect Donald Trump hasn't even been sworn into office, but at least one polling firm is already looking ahead to the next presidential contest.  What Public Policy Polling found is this: A severe discrepancy between what Democratic primary voters want, and what's currently available. 

A majority of surveyed voters said they would prefer it if a candidate younger than 60 or 70 years is nominated in 2020, according to a survey published Tuesday.  A plurality (41 percent) of these same voters also said they want someone who has never run for president before. 

In short, Democratic primary voters want a younger, fresher face to represent them in the general election four years from now. 

However, when it comes to the party's possible 2020 bench, the three leading candidates named in the PPP survey will all be in their 70s by then, and two have already run for president. 

Vice President Joe Biden is the leading contender in the new PPP survey, which was conducted between Dec. 6 and 7, drawing a full 31 percent of support from surveyed Democratic primary voters. 

Sen. Bernie Sanders, I-Vt.,  came in second with 24 percent, and Sen. Elizabeth Warren, D-Mass., came in third with 16 percent. 

By inauguration day 2021, Biden will be 77 years-old.  Sanders will be 78, and Warren will be 75.  Unless Democrats starts grooming younger candidates soon, the average age of the party's supposed bench in 2020 will be about 70. 

At this rate, the party may as well float Walter Mondale as a potential candidate.  He will be 92-years-old in four years. 

Biden, Sanders and Warren are "the only folks we tested with meaningful support for the nomination at this point," PPP explained. 

They also polled Democratic primary voters on their support for some of the party's younger stars, but the results weren't even close to the sort of response PPP saw for Biden, Sanders and Warren. 

Sen. Cory Booker, D-N.J., got only four percent of support from surveyed voters.  Sen. Al Franken, D-Minn., and Kirsten Gillibrand, D-N.Y., each got three percent.  Sen. Sherrod Brown, D-Ohio, and New York Gov. Andrew Cuomo came in with only two percent of the vote, and Julian Castro, who heads the U.S. Department of Housing and Urban Development, got less than one percent. 

Booker will be 50-years-old in 2020.  Franken will be 68-years-old, Gillibrand will be 53, Brown 67, Cuomo 62, and Castro 45-years-old. 

"There's a tension for Democrats nationally," said Dean Debnam, president of Public Policy Polling. " They want new blood, but their most well known and popular figures don't exactly fit that mold.  Of course they have a long time to get that all figured out." 

The PPP survey polled 400 Democratic primary voters and has a margin of error of plus or minus five points.