I don't know how to use Register to find out who is assigned to a specific role, only to assign them to empty roles. But resort workers are service sims anyway. I used MC to check the personal status of all resort workers. There are five of them, probably because I repeatedly tried hiring a different one for La Costa Verde, all to no avail. Three of the resort workers are young adults, but their age is listed as 179 days, which is the last day of the teen stage in this game. One of them is listed as an adult but is at the last day of the young adult stage.
The last one is a young adult at 180 days. She's the one who is manning the desk at Hobart's, which I did have my sim buy, on this same sim day as it is saved at now. And yes, she is right where she is supposed to be, at that desk, and there's a maintenance worker on site with her.
So it appears that you are correct, that they are stuck aging up. But not being role sims, I can't use register to assign them. I tried making a new sim, placing him in the game, and assigning him to the service pool. But when I clicked on the desk to hire someone, he wasn't listed in the options. So I annihilated all of the sims in the pool who were apparently stuck aging and tried again. He still wasn't among the options. The game had generated three new resort workers, all of whom were adults at the last day of the young adult stage. So instead, I used MC to assign an absolute age to one of them, of 10 days into the adult stage, went back into buy and hired her. I don't have time to play now, but when I do play tonight, I'll find out whether that worked.