Dr. Jeremy Brown, in a postpost on his Talmudology blog on science in the Daf Yomi, points out that Rava, quoted in Yevamot 97a, provides the first published claim that boys' puberty can be delayed by their being either overweight or underweight.
כי אתו לקמיה דרבא אי כחוש אמר להו זילו אבריוהו ואי בריא אמר להו זילו אכחשוהו דהני סימנין זמנין דנתרי מחמת כחישותא וזמנין דנתרי מחמת בריותא
Whenever people came [with such a case]* before Raba, he used to tell them, if [the youth was] emaciated, ‘Let him first be fattened’; and if he was stout, he used to tell them, ‘Let him first be made to lose weight’; for these symptoms disappear sometimes as a result of emaciation and sometimes they disappear as a result of stoutness.
* Of one who reached the age of twenty without having produced two hairs.
(Translation and footnote from Soncino [PDF])
Dr. Brown points out that these associations have only been confirmed in the scientific literature in the past fifteen years, citing two papers as the first published confirmations of excessive weight and insufficient weight, respectively, being associated with delayed puberty in boys:
Lee, Joyce M., et al. "Body mass index and timing of pubertal initiation in boys." Archives of pediatrics & adolescent medicine 164.2 (2010): 139-144.
He, Qing, and Johan Karlberg. "BMI in childhood and its association with height gain, timing of puberty, and final height." Pediatric research 49.2 (2001): 244-251.
(Hat-tip to Rationalist Judaism.)