Love between a young adult and a teenager is prohibited

Just age both Sims up at the same time. 

Yep I know but that means that you can not fall in love with a non-played sim because they usually don't age.

I'm not sure what else to suggest. The Sims Team are not going to allow for romance between teens and young adults. So my suggestion is to simply age up the other Sim when your Sim ages up and continue the relationship from there.

That would be extremely problematic... I understand the frustration but the idea of "Young adult" could be anywhere from 18-25 and a "teenager" could be anywhere from 13-19 and while theres a bit of overlap between those there is also a huge age cap for others. CTRL shift C cas.fulleditmode - shift click on desired sim - edit in cas - edit age to desired range. This is the solution. 

Also non playable sims most certainly do age up... just most likely not at the same time as your played sim.

Whenever I have a Sim that needs to age up along with my Sim I invite them to the birthday and make them blow out the candles on the cake as well.

Hi, I have a young teenage couple who chose to get engaged. One of them is coming of age before the other, what happens then?
Thank you.

Hello 😊 when you have two teens who are Promised to each other and one of them ages up. They will remain Promised to each other but you must make sure to age up the other Sim as well. Untill you do so, any romance options will be unavailable.

Okay, thank you.

Of course, no problem 😊