There is a lot of confusion about this topic. It's generally accepted that babies need a pediatrician, and later in life people with medical problems should see a doctor regularly. But the reality is that EVERYONE should have a primary care doctor!
Historically, many people had their family doctor that took care of the whole family. In small towns, there's often one doctor in town that cares for the whole population. Other times, the children in a family see a pediatrician, while their parents and grandparents see a family doctor or an internist (internal medicine physician). Adults with several medical problems usually see an internist, as they have additional training in managing complex patients.
Unfortunately, one major problem with this system is when people graduate from their pediatrician's care, there is often a big gap before they establish with a primary care doctor as an adult. While it is true that many adults in their 20s and 30s are healthy, so much can be missed by not going to a doctor regularly. Medical problems such as anxiety, depression, high blood pressure, migraines are common in this age group, and should be diagnosed and treated quickly to enable young adults to thrive in school, work, and their lives. This is also a time where review and adjustment of healthy habits is vital.
In the 20s and beyond, men and women often find themselves putting other things ahead of their own health: namely their children, school, and careers. I remember when I was in medical school and a wise attending told me, "I'm too busy NOT to exercise!" The same concept applies to our health: if we want to keep doing the things that are important to us, be around to enjoy our children and grandchildren, and see the fruits of our labors at work, we can't put off taking care of ourselves. Regular check-ups and frequent discussions about healthy diet and exercise can help ensure you stay physically, mentally, and emotionally well throughout your life.
As we age, many elderly patients dread going to the doctor fearing a problem will be found or they'll be asked to take one more medication. I encourage older patients to be proactive in seeing their doctor, and ask the tough questions like, "What can I do to stay healthy and active?" and "What changes can I make to continue to live independently?" Make sure you have received all of the recommended health screenings and vaccines. Don't wait until an acute illness strikes that leaves you scrambling to catch up.
Finally, when healthcare concerns arise, you should know immediately who you're going to call for help. (And I hope you're not thinking of Dr. Google!) If your grandfather is diagnosed with cancer, you should be able to meet with your primary care doctor to discuss your own risks and whether you need additional screening. If your toddler seems to be limping, you need to get in to see her primary care doctor right away. When you develop that weird pain that you can't make sense of, you should have your primary care doctor on speed dial to make sure it's nothing serious. Otherwise, after silently stressing for weeks or more, you'll likely end up in an emergency room, possibly getting a lot of unnecessary tests and definitely getting stuck with a huge bill.
I could go on and on. Things happen. Life happens. Get a primary care doctor so you know who you're going to call when it does.