Scientists used to believe you had all the brain cells you were ever going to have when you were born. And that your brain never grew or changed. But recent studies show your brain does change at different stages in life. So, here are the decade-by-decade mental shifts that prove it:

• In your teens, you probably didn't care about the future that much. But in your 20s, you start to focus on it. And according to the researchers at Berkeley, when 20 year olds become more mentally and emotionally mature, it gives them a huge brain boost when it comes to organizational ability, and the discipline to put plans into action.
  
• At 30, we become natural nurturers. According to the UC Berkeley scientists, as we get into this stage, we become much more helpful, warm and generous. One contributing factor is an increase in the hormone oxytocin the bonding hormone. The result: At 30 you're more grounded, and better able to pay attention to others.
  
• By the time you hit 40, you're unstoppable. Your body boosts production of high-energy hormones, like testosterone, so nothing gets in your way. Not even the common cold! In fact, studies show people are a third less likely to get a cold after age 45   about the same time their earning power peaks.
  
• So, how does your brain-change at 50? You become a self-assured risk-taker. UCLA researchers identified a correlation between making it past the half-century mark and greater self-confidence. You feel freer to follow your heart and are less worried about what other people think.
  
• Finally, 60 and 70-year-olds are unflagging optimists. The experts at Fordham University say since there are fewer stress hormones in your system as you age, you become more cheerful and optimistic. We also learn from experience what makes us truly happy by that age.

If you'd like to go further, check out the book Brain Builders!: A Lifelong Guide to Sharper Thinking, Better Memory, and an Age-Proof Mind.