I believe that we are definitely healthier in the 21st century because we have the right resources to stay healthy. The previous centuries have not correctly understood what they have needed to do in order to stay healthy, but now we have the correct information and resources to allow us to remain healthy, meaning that we are healthier in the 21st century.
In first world countries, health has been improving for decades. In developing nations, new technologies introduce medical hazards not present in an agrarian society, so health has declined. It depends on who you are. Even then, it isn't black and white. Cancer is on the rise worldwide, but donations to cure diseases and relief funds make people better. All in all, health is slowly winning.
I believe that we are healthier in the 21st century. We have vaccinations that can wipe out serious diseases. Even though we might not always follow it, we have a good understanding of what comprises a healthy diet-the importance of fruits and vegetables, less red meat, etc. Great strides are being made in the fight against cancer and the prevention of the disease.
Yes, we have started eating more organic foods, and staying away from red meat, but there is a lot more to being healthy. People use their cars to run to the store, two blocks away. They drive their children to the bus stop in the morning. We don't have night time strolls, and when we order out, we supersize everything. On top of that, more people are smoking, and at a younger age.