Is it rare for someone who only holds an undergraduate degree to apply and be accepted to PhD programs? Just want to know the boards opinion or if anyone has done this? Do most mid level PhD programs expect you to have some sort of Masters/MBA degree?
one of the phd students in our department had only undergraduate degree (and few years of work experience). so i guess it is possible - depending on other factors. I think that harvard somewhat encourages going to grad school as early as posible. their hbs average phd student (at the start of the program) is reported to be 26 in the past 2 years. so there isn't much room for prior graduate before starting your phd. I guess that some of those "below the average age", came fresh from college, with or without 1-2 years of work experience.
but this is just a guess.