Charlie tells Alan that he (Charlie) isn't middle-aged.
At the time this was filmed, Charlie was 44 years old, and middle-age is usually considered to start at age 40.
Alan says that he is younger than his brother Charlie (something previously established on the show), but in reality the two actors (Charlie Sheen and Jon Cryer) were born in the same year (1965). However, Jon Cryer was actually born a few months before Charlie Sheen, making Jon technically the older of the two.
Episode title spoken by Charlie, to Alan.