By Dawn Harris Sherling, M.D., F.A.C.P.

In the past week you have probably come across an article about a Hollywood star having kids in her 40s. You may have a friend who headed back to school and changed careers in their 40s, or at the very least, know of a friend of a friend. In the era of Botox and Viagra, you can look and perform like a 20-year old well into your 30s, 40s, and beyond. It is getting hard to tell who’s 40 anymore. However, it is often said that you are the age that you feel you are. In that vein, I have compiled a helpful list to determine your “real” age:

If you wake up with back pain for no reason…you might be 40.

If you develop sudden knee pain while doing nothing more strenuous than climbing the stairs…you might be 40.

If you eat pretty healthfully, exercise occasionally, and still gain 2-5 pounds a year…you might be 40.

If you start finding hairs in places you didn’t know hair usually grew…you might be 40.

If you are wide awake at 2 a.m. after having an extra glass of wine at dinner…you might be 40.

I’ll stop here, but this list could be a lot longer. The truth is, the body ages. There are things like smoking, heavy drinking, drug use, leading a sedentary life, and being obese that age the blood vessels, brain, lungs, and joints faster, but we all age. I am prone to remind people that this is better than the alternative, but as a patient once retorted, I can’t really say that for sure.

Now in my 40s, I can safely say that I expected these symptoms to appear later. Maybe in my 50s or 60s? I’m not sure when, but certainly, not in my 40s—in my prime! The truth is, I find myself saying the same words I heard from patients for many years, “Why is _____ happening? I’m not doing anything different!”

And therein lies the problem. It turns out that when you are 40 (or older), when you have reached the point in life where you have finally become comfortable with who you are, you will have to change if you want things to stay the same (or as close to the same as possible). That means doing a lot more core work if you want to stave off back pain and keeping the muscles toned. You need to get strong and stay flexible. Yoga works for many people, but there are lots of exercise programs out there that target strength and flexibility as well.

Mostly, and most challenging, we need to fight weight gain. Metabolism starts to really slow down in our 40s and continues to slow as we get older. That means we need to do more physical activity and eat less than we did in our 20s just to maintain the same weight. (Please don’t send me hate mail—I don’t make the rules, just trying to live with them).

There are magic creams for wrinkles, my hair colorist is a genius, and Spanx is one of the greatest inventions of the 21st century. I think I look younger than my 42 years. But if I miss a week of yoga or have an extra glass of wine or two, my aches and insomnia will be there to remind me to get back on track. 40 can be the new 20, but, sadly, only sometimes.

Dr. Sherling is a general internist and assistant professor at Florida Atlantic University Schmidt College of Medicine. She is the author of several academic articles, a book chapter on diabetes prevention, and a medical thriller, Not Quite Dead.

Website: dawnharrissherling.com

Twitter: @dharrissherling