The former is skippered by 22-year-old yachting prodigy Peter Burling and the latter by the aptly-named William Tiller. There are also teams from Germany, France, Australia, Portugal, Switzerland and Sweden (the Swedish Youth Challenge, attached to Cup challengers Artemis Racing).
The what-could-have been sense perhaps stems from Oracle CEO Sir Russell Coutts' statement last year. He said then that, with the considerable advantage of hindsight, the 34th America's Cup could have been raced in boats more like the AC45s - the smaller but still fast and exciting 45-foot catamarans used in the build-up to the Cup, the America's Cup World Series raced at a series of international venues and won by Oracle.
"In hindsight, I think there were two errors. One was I thought the boats needed to be quite large-scale to be grand enough for the America's Cup. Clearly the World Series has proven this wrong - the AC45s look pretty damn good on TV," he said. "The other thing is, we possibly should have looked at making more of the components one-design."
The rules surrounding the Youth Cup are quite strict. Not only nationality- and age-restricted but the crews cannot begin using the AC45s until August 12 - not long to get used to the boats in the difficult tides and currents that are part of racing on San Francisco Bay.
That's because the organisers are keen to ensure there is no local advantage gained by the US sailors - though America Youth Sailing Force and USA45 Racing must be the teams to beat after having had use of Oracle's two AC45s previously. All the AC45s, with their 20m wingsails, have now been handed over to race organisers and will not be released to the team until August 12.
Ian Andrewes, of AYSF, said: "Having access to Oracle has been a huge advantage for us; we have had the AC45s out for a significant amount of time on the water."
Burling, who with partner Blair Tuke, won a silver medal at the 2012 Olympics in the 49er class behind Australia's Nathan Outteridge (now with the Artemis America's Cup challenge), heads the NZL Sailing seven-man team, also including Tuke, international Laser sailors Sam Meech and Andy Maloney, multihull sailor Jason Saunders, Jono Spurdle and Guy Endean.
The Full Metal Jacket team includes Tiller, one of the brightest young match racers around, plus Bard Farrand, Harry Thurston, Matt Steven, Shaun Mason, Stu Dodson and Ash Hammond.
The fleet racing should be spectacular.
The Youth Cup will be held in the interval between the Louis Vuitton regatta and the America's Cup match race for the trophy - already strongly favoured to be between Oracle and Emirates Team NZ.