My DS was 8 on our last trip, and begged to not wear his "If my parents are lost..." bracelet. He told us he was a big boy and knew our phone number by heart (which he does), so if he got separated from us, he'd know to look for a cast member and ask them to call my cell. I flat out refused to let him get off of wearing the bracelet and used the reasoning "What if you got hit in the head and you couldn't talk?" DS grudgingly wore the bracelet after that, and boy am I glad I stuck to my guns!
DS got separated from us at The Seas in Epcot, and I never thought about how huge the observation level really is until DH was running all over trying to find our son (I parked myself at the exit in case DS didn't know better and left to see if we were outside). Five minutes after I told a cast member DS was lost, I got a call on my cell phone from another cast member saying that he had found DS huddled in the "Shark Room." DS was crying hysterically and there was no way he would have been able to remember my cell phone number in his state of mind - when the cast member approached him, he just stuck out his "If my parents are lost..." bracelet. The CMs thought it was the best thing they had ever seen and mentioned that too many people can forget basic things in the heat of the moment...
I had also taken a pic of DS that morning and showed it to the CM when I reported my son lost, since I couldn't remember what he was wearing at the time. When the CM announced the lost child on the radio, she said that my son was wearing an orange shirt with a bear face on it. The CM that found my son was from Chicago and knew instantly that DS was wearing a Chicago Bears shirt - he said it made it a lot easier to find him, since he knew which shade of orange to look for...
So, to make a long story short (too late), yes, I'd either use the bracelet or some other identifier, even if you think they're too old. I hope that they wouldn't get separated, but if they do, and God forbid something happens to them where they can't talk, someone'll need to get a hold of you...better to be safe than sorry...