Accompanied by support runner Ryan Truzzi-Falconi, Mr Stokes is setting off from Spean Bridge in the Scottish Highlands, before running south-east through Europe as far as Turkey.
He will then run east through Saudi Arabia and the Middle East, before running through Malaysia, Australia and New Zealand.
From there, the challenge will take him across North and South America, to a brief stop in Iceland before he runs north from Spain and finishes back at Spean Bridge.
Mr Stokes is aiming to raise £100,000 for the RNRMC, which supports serving Navy personnel, veterans, their families and distributes money to other military charities.
Dawn Ingram, head of fundraising, said the charity was “incredibly proud”.
“Every mile Fred and the team run will help us be there for the naval community for life,” she said.
The current record-holder for the youngest person to run around the world is Danish athlete Jesper Kenn Olsen, who completed 16,307 miles (26,243 km) in 2005, according to the WRA.