In 2001, the Word Health Organization recommended that babies be exclusively breastfed for 6 months, after which they should gradually be offered complementary foods until the majority of their diet consists of solid foods at around a year old. This replaced their earlier recommendation of exclusive breastfeeding for 4-6 months, after it was shown that babies nursed exclusively for the whole 6 months had fewer episodes of gastrointestinal disease, and their mothers benefited from greater weight loss postpartum and better birth control due to lactational amenorrhea. While also universally adopted by various national health organizations, some researchers maintain the evidence for this change was insufficient, doubting the capability of breastmilk to adequately nourish some infants until the age of 6 months in developed countries, where infectious disease is of relatively lesser concern (see here and here). These concerns are somewhat diminished by the recommendations to supplement all babies with vitamin D and iron, however.
However, it seems to be popular among the AP/NP set to delay the introduction of solid foods even past the six-month mark, some even past the first year. There seem to be multiple reasons for this: