Your real age is determined by the date on your birth certificate. The age you feel is another matter entirely.

Some of us start to feel creaky and absent minded well before retirement. Another lucky bunch, memorably dubbed "superagers" by scientists, retain the clarity of mind of someone in their 50s well into their 80s. What sets these groups apart?

That's, of course, a hugely complicated question, with genetics, levels of physical fitness, diet, life experience, and the luck of the draw when it comes to illness clearly playing a huge role. But while there are many factors you can't control, recent research suggests there is one way to increase your chances of becoming a superager that's entirely up to you: Just maintain close friendships.

Your brain matters, but so does your book club.

Scientists at the Northwestern University Feinberg School of Medicine have been studying superagers for years, with their previous work looking at the physical differences between these awesomely slow agers and your average 80-something year old. For their latest study, however, the team took a different approach.

This time, rather than run volunteers through an MRI, they had them answer a standard questionnaire that measures psychological well-being by asking about things like feelings of mastery and competence and social relationships. The results show that it isn't only superagers' brains that look different from those of regular seniors--so do their social calendars.

Compared to those aging at an average pace, the superagers reported having significantly more close friends. "You don't have to be the life of the party, but this study supports the theory that maintaining strong social networks seems to be linked to slower cognitive decline," study author Emily Rogalski commented in the study release.

Of course, organizing a weekly card game or book club with your best buddies can't guarantee you won't get Alzheimer's, but given that you'll probably enjoy the meet-up anyway, it's good to know that science suggests something so simple at least increases your odds of keeping your brain sharp as you age. (Previous studies have suggested you might also want to mix in some mindfulness and weight lifting).

Friends are a miracle drug at any age.

And this advice shouldn't just be of interest to those approaching the far side of middle age. Study after study has found that maintaining warm, close friendships has outsize health benefits at any age. Hanging out with good friends is basically a miracle drug, counteracting the health toll of loneliness (obviously) and possibly adding years to your life.

So don't wait until your hair turns gray to start. If you struggle to make time to maintain your friendships in the crush of middle age mid-career craziness, just remind yourself that socializing isn't an indulgence. It's as much of a health essential as nutritious meals and hitting the gym.

When was the last time you found time to hang out with your best friends?