I'm not sure why the reliable and functional mechanics from the Sims 3 were replaced with an unnecessarily complex mechanic that works sporadically and that you never seem to fix. Right now I have toddlers that I can't age up. My sim baked literally every cake from both cooking skills (which are both maxed) and none of them have an option to add candles. Every time I have a birthday it takes 5-6 attempts to get a cake that works. I'm not exaggerating. Every time.
Trust me. Just buying the cakes from build/buy in TS3 was more fun and more reliable. Even if this silly mechanic actually worked it would add nothing to the gameplay. Plus in the Sims 3 we had multiple options to age up Sims. Now we have one, and it rarely works. You've fixed it several times in the patch notes, BUT YOU NEVER FIX IT IN THE GAME!!!!!! It always glitches out again.
Please give us reliable methods for aging up sims. Please give us more than one. Please. Please. Please. Make it work reliably. I'd really love to just buy birthday cakes in buy mode again. Seriously. It was better.