While it’s easy to see this as mostly correlation, there probably is some moderate causation.
There’s a higher maternal death rate for girls who become pregnant at or shortly after puberty, and such pregnancies can degrade health otherwise (an undersized girl doesn’t carry to term well, can suffer gestational diabetes with long-term adverse effect, and abortion has health risks too). Early sexual activity directly increases STD rates, and HPV (cervical cancer), AIDS and syphilis can kill you directly and dramatically can increase all-cause morbidity and mortality, while STDs that are generally not fatal (gonorrhea, herpes) are nevertheless chronic and increase all-cause morbidity and mortality from the degradation of overall health.
Well, if you read the whole thing, it sounds like they tried to factor out those things.