One of the measures used in studying health and obesity is the Body Mass Index (BMI). A BMI measure between 20 and 25 is considered healthy; a measure of over 30 makes a person obese. The average BMI in the United States rose from 25 to about 27.75 in the last 25 years. This is considered a large increase, and many social scientists have been trying to explain this trend. The results below come from a regression estimated using a national cross section sample of adults. The dependent variable was BMI, and the sample size was over 42000. Independent Variable Sample Mean of Variable (Standard Deviation) Coefficient in Regression (Standard Error) Restaurants per capita in respondent's state 11.04 (2.27) .473 (.324) Age in years 41.24 (16.68) .326 (.016) Age squared -- -.003 (.0002) Cigarette tax per pack in cents in respondent's state 22.58 (9.77) .058 (.032) Cigarette tax squared -- -.001 (.0004) Household income in 10,000s of dollars 2.91 (2.39) -.393 (.168) a. According to this regression, at about what age does BMI reach its highest level in American adults, other things equal?