YOU SAID:

so I heard that dogs are supposed to die when they turn fifteen

INTO JAPANESE

犬は15歳になったら死ぬはずだと聞いた

BACK INTO ENGLISH

I heard that dogs should die when they are 15

INTO JAPANESE

15歳の時に犬は死ぬべきだと聞いた

BACK INTO ENGLISH

I heard that the dog should die when he was 15

INTO JAPANESE

犬は15歳の時に死ぬはずだと聞いた

BACK INTO ENGLISH

I heard that the dog should die when he was 15