First off before they can mate they have to be adults. Second, the 4 smiles can go up to be 2 hearts and 2 smiles which means best friends and it can go higher to four hearts, which means partner.
Also, im not sure if this is always what happens but my two tamas were an adult boy and a teen girl and my situation was pretty much the same except that te adult went up to four hearts and the girl only had four smiles but the next day the girl was an adult an it went up to four hearts. So im not sure but maybe the teens cant go any higher than four smiles until there adults. But like i said im not sure. hope that helps