The 3 or 4 day cruises (i.e, Fascination to Bahamas) are where you will find the most young people. However no ship will be "mostly 20's and 30's". Many people in that age group do not have the disposable cash to cruise on a regular basis as compared to retired folks, families and empty-nesters.
General rule, the shorter (cheaper) the cruise, the younger the passengers.
You may want to try one of the cyber travel group cruises. Vacationstogo.com, cruisingforlove.com, singlescruises.com. Do not let the group's names deter you because it is more about enjoying the vacation in a group setting than dating, matching or hooking up.
Good luck and
I B CRUZIN'
(solo cruzin' by design)
CCL Victory, CCL Imagination, CCL Victory
Past Cruzes: RCCL Navigator, RCCL Majesty, CCL Destiny,
Glory (2007),Liberty (2007), Fantasy (2007), Destiny (2006), PofAm (2005), Inspiration (2005), Fascination (2004), Victory (2004), Fascination (2004), RCCL VOS(2003), Fantasy (1999)