Can young adults marry? Can they marry an adult sim?

So I had two sims, one young adult and one adult. I did the wedding ceremonies and eloping and the married status didn't appear. I made the young adult become adult and made them eloping and it worked. 

So why is it that? 

Bug?

Young adults can't marry? 

Sims of different ages can't marry? 

Thanks 😊✨️✨️✨️