Cakeage is not an exciting word, or concept, despite relating to the pleasant subject of cake. It refers to the fee charged by a restaurant when you bring in a cake from outside, to serve in said restaurant. You may be wondering what occasions might require such an act, usually this happens on birthdays, though leaving dos/retirement parties/wedding receptions might also involve cake. In fact, any good party should involve cake.
Mind you, I’m struggling to think of an occasion where I, or anyone I know, has been charged cakeage. I don’t doubt that it happens, and I can understand why a restaurant would not be pleased if everyone started bringing in their own desserts, but as a rule, who is so miserly that they’d charge a fee for say, a ten year-old’s birthday cake? That seems a bit cold-hearted to me.