Welcome to cruise-chat.
Age limits are adhered to, because if they start making exceptions because kid X is only 7 weeks away from being 3 years old then they end up compromising the program. You can't effectively manage a youth program if you start making exceptions. How soon before it becomes a "you let that kid in, why not mine" battle?
The youth staff gets a manifest of the names and ages of all kids on the cruise. It won't hurt to ask and perhaps someone might overlook it, but I wouldn't expect things to go your way.