If you plan to get married and have kids like most people, who are in their late 20s and early 30s, do, then I think it's better to do the residency right after dental school and start your career as early as possible. Being able to earn 6-figure income at a younger age gives you more flexibility later in life. For example, If your set up a practice and it fails, you have time correct the failure...ie relocate to a less saturated area and start another practice. It's also good to invest your hard earned money early. If you lose all your investments early in life, you still have time to fix it. When you and your spouse haven't had kids yet or have kids but they are still young, you have more flexibility to travel far to many different offices and make a lot of money. When your kids reach their teenage years and need more attention from you, you can afford to work less so you can spend time with your kids. Keep in mind that you won't be in the same physical shape 15-20 years from now, no matter how regularly you exercise. I know plenty of people, who are in their 40s and 50s, start getting things like back pain, high blood pressure, poorer vision, carpal tunnel, high cholesterol, diabetes, arthritis etc. You don't just get back pain from practicing dentistry.
http://www.aans.org/Patient Information/Conditions and Treatments/Low Back Pain.aspx
If you plan to stay single for the rest of your life, then you can do whatever you want and retire whenever you want. Kids are expensive. Paying off student loan is just a small part. There are a lot of things you have to worry about such as home mortgage, business loans, kids' college tuitions, health insurance, 401k, and other investments for your future retirement (can't work forever) etc. I know there are plenty of people here, who are older, have kids, and still want go back to school to specialize. I really admire those people. I think that it's easier to get more things done when you are young and healthy. That's why I always tell many young high school and college kids not to mess around. Stay focused, pick the school with the cheapest tuition, and finish school on time.