Is it true that if your below a bmi of 18, 5-20 your lifespan might be shortened? i'm naturally very slim 17bmi bc i'm 6ft tall it's very hard to gain

It's hard to know : Your bmi is low. I think a lot depends upon why your bmi is low. If many family members have low bmi and they are long-lived, there is probably little reason to worry. On the other hand, if you have other health conditions affecting your weight such as uncontrolled diabetes, substance abuse, eating disorders, or other chronic diseases, it is probably those disease rather than the bmi itself.