Long term effects of low testosterone levels for a 19 year old?

Higher mortality. In short term, low t has been associated w/low bone mass & fracture, frailty & sarcopenia (loss of muscle), obesity, diabetes, metabolic syndrome, depression, loss of libido, erectile dysfunction & more. Years later if untreated, worry about higher all-cause (gold standard of all studies), cardiovascular & cancer mortality (death rate) as shown in 7 different populations across the globe.