Honestly that sounds like a bug. Blow out candles and make a wish are both actions that should always age up the Sim doing them, as far as I know. When you hover over the option, does it give you the warning for it? Something like "This will age up your Sim"?
It can be difficult to actually get to the blowing out candles stage - you need a new, uncut cake, and it needs to be placed exactly in the center of a table or counter before Sims will put candles on it. Sounds simple, but becomes problematic when depending on a caterer who keeps putting things down on the edge of the table or just making taco casseroles. And if you make the cake ahead of time, you need to guard it to make sure none of your Sims take a piece. And then the birthday Sim needs to actually get to it, and of course all the guests will be standing in the way... But once the candles are blown out? I've never had a problem. It should work even without the party.
Yes, I am getting the warning message, the candles are on the cake, it's uneaten/uncut and I moved it to the center of the table.
