Actually,all UKISS members and all kiss mes know that Xander was really born in the year 1988 but what's written in some ukiss infos is 1989 wherein Xander's parents had registered Xander in that year. In short,just consider Xander as the real oldest ukiss mem and not SooHyun,because he is! :)
Well, what you said is true that on shows they say that Alexander is the oldest and that's true. They probably say Soohyun is the oldest because without Alexander and Kim Kibum he's now the oldest one in U-KISS. SO to summarize Alexander was the oldest, but with him gone and out of U-KISS Soohyun , the leader, is the oldest now.
if xander didn't leave the group, then he would've still been the oldest. soohyun is now the oldest. you might have been confused because before when xander was there, there was no leader. but then i guess they decided that soohyun should be...