These things just kind of sneak up on you and suddenly you notice..."Hey, Freckles has tail streamers".
Because "he" is so huge, it is easy to forget that Freckles is still a baby. He has not yet been in the tank for 2 years. (2 years in Jan, 2010) It seems that shortly before 2 years old, he suddenly developed tail streamers confirming that he is, in fact, a he.
The timing is about right, IME. I added Moe (the blonde Naso) almost 2.5 years ago and when I purchased him he was at least 1 year old judging from his size. He first began to show streamers about 1 year after purchase. Now, at about 3.5 years old, his streamers are just getting more stunning all the time.
So....is my youngest next? Lance (the unicorn), has only been in the tank for 18 mohths and was a small juvenile when purchased. I don't even know if Unicorn Tangs develop tail streamers but his tail looks like "he" might. I could get get lucky for a 3rd time!

__________________
400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank:
http://www.canreef.com/vbulletin/showthread.php?t=28436