Final answer:
Infants are typically ready for nipple feeding around 34-37 weeks of gestational age due to the development of sufficient sucking and swallowing reflexes. A hypothesis test with sample data showing a mean weaning age of nine months suggests that the mean weaning age in the US is significantly less than the global average of four years old.
Step-by-step explanation:
The question of whether nipple feeding is likely to be successful at a certain gestational age refers to the readiness of an infant to feed from the breast or bottle. Infants tend to be ready for nipple feeding by around a gestational age of 34-37 weeks, as they have developed sufficient sucking and swallowing reflexes. Prior to this age, infants may not be fully capable of coordinating the suckle reflex, which is critical for successful feeding.
Regarding the hypothesis test to determine if the mean weaning age in the United States is less than four years old, we would set up a one-sample t-test with the null hypothesis stating that the mean weaning age is equal to or greater than four years (48 months) and the alternative hypothesis suggesting that the mean weaning age is less than four years. Given the sample data with a mean of nine months and a standard deviation of four months, the t-test would likely reject the null hypothesis, indicating that the mean weaning age in the United States is significantly less than four years old, especially considering the large difference between the sample mean and the hypothesized population mean of 48 months.
It is important to note that the readiness for nipple feeding depends on individual developmental progress and may vary among infants, but typically, it is successful when attempted after the gestational age of 34 weeks.