It appears a lot of the teams last year basically took their youth team along. I don't know if that's the idea or its clubs not taking it too seriously.
http://www.hksoccersevens.com/teams/main
It's a youth tournament for the overseas teams - but the local Hong Kong clubs field teams with senior players (and struggle against the foreign youth sides - HK teams have only won the cup twice). It also includes the Hong Kong FA u-21 and u-23 rep sides.
Man City last year sent their u-18 team - all their players aged 16-18.
Some oreign clubs send a couple of 20-21 year olds with the teens.
The oldest players in the foreign sides seem to be about 21 years but most are in the 17-20 age bracket.
Some future big stars have played in it:
"Aston Villa remain the undisputed Soccer Sevens kings, with five victories and two other final appearances since the tournament was first held in 1999. Their most recent success came in 2010 when, inspired by two players with first-team experience, striker Nathan Delfouneso and winger Marc Albrighton, they beat Rangers 3-2 in a thrilling final.
Numerous Villa youngsters who have appeared in Hong Kong have gone on to make their mark at senior level, including the likes of Gabriel Agbonlahor, Craig Gardner, Liam Ridgewell and Gary Cahill in years past and, more recently, Delfouneso, Albrighton, Barry Bannan, Cairan Clark, Nathan Baker and Andreas Weimann."