Now! What i mean is that if you think you have hypogonadism (low testosterone), there is no reason to wait for a specific age before asking your doctor to evaluate & treat you if appropriate. There are many reasons for younger men to develop hypogonadism & there is no reason to suffer nor risk worse health. So if you're worried, talk to your family physician now!
Around puberty age. Boys who don't have evidence of puberty by age 13-14 should be evaluated by their doctors. A pediatric endocrinologist can help. If boys are known to have hypogonadism for any reason, they will need testosterone supplementation to go thru puberty. Not infrequently, constitutional delay of puberty may also require a jump start with low dose testosterone for some months.