Popular culture and Internet listicles often portray the 30s as life's best years. Free from the financial and personal insecurity of the 20s and not yet approaching the midlife challenges of the 40s and 50s, the 30s are said to be the best of both freedom, and responsibility. But what does science say about being a 30-something?

Research presents a mixed picture of a decade of life marked by increasing stability as well as significant change. Some studies suggest that 35 is the "best age" and that real happiness begins at age 33. People older than 100 years in overwhelming numbers regard their 30s as being the best decade of their lives.

Still, the 30s have also been found to be a time of existential crises, ticking biological clocks, and heightened job dissatisfaction.

Here's what science has to say about the ups and downs of being a 30-something:

The beginning and end of the decade may be marked by significant life changes.

If you're going to make a major career change, move to a new city, run a marathon, or have an affair, you're most likely to do it when you're about to turn 30.

Those entering or leaving their 30s are likely to conduct a sort of "life audit" to assess meaningfulness and satisfaction. We tend to use the bookends of a decade as opportunities to evaluate our life paths, and to make changes, according to recent research. New decades tend to inspire a search for meaning, and may lead us to "imagine entering a new epoch," said the researchers, who observed the behavior of "9-ers" (those aged 29, 39, 49, etc.).

You may hit your sexual peak ...

One big thing to enjoy about being in your 30s? Great sex.

For women, a ticking biological clock may be a downside of progressing through the 30s. Perhaps because of this phenomenon, women reach a sexual peak at this time of life, according to research. Women in their 30s and early-40s are significantly more sexual than younger or older women, reporting more sexual fantasies and more actual sex. The researchers hypothesized that women experience enhanced sexual motivation and behavior as an evolutionary adaptation that would have led them to capitalize on their remaining fertility.

Whether this is the actual reason, many women in their 30s say they feel sexier and more in tune with their bodies -- and therefore enjoy a better sex life -- than they did in their 20s. At age 31, women are their most sexually confident, according to a survey reported by the Daily Mail.

... And soar to new heights in your career.

While the 20s are generally characterized by completing your education, unemployment or underemployment, choosing a career path, and working long hours to move up the ladder, the decade that follows is more about enjoying career success and financial success.

The ages of 30 to 39 can be a time of career highlights. Thirty-something women can look forward to pay growth peaking at an average age of 39, according to a Payscale.com analysis. And if you're an artist or a scientist, you'll be most likely to have your biggest creative breakthrough in your late-30s, according to a study of scientific innovators and Nobel Prize winners. A 1977 study, cited by The Atlantic, found that physics Nobel winners were an average of 36 years old when they did their prize-winning research, while chemistry prize winners were an average of 39 years old.

If you're not happy with the career path you've chosen, you're likely to feel worse about work. Some research has shown that 30-somethings are less satisfied with their jobsand more emotionally burnt out than people in their 20s and 40s.

Your personality probably won't change much.

The 20th century Harvard psychologist William James said that after age 30, the personality has "set like plaster." James believed that personality tends to stabilize with the emergence of adulthood. Some research backs up this early belief.

Our core personality characteristics are at least partially determined by genetics. But from childhood through the 20s, our personalities are evolving significantly, and these changes slow as we approach 30. While our fundamental personality traits don't change much once we hit the big 3-0, that doesn't mean we can't challenge ourselves, act out of character and grow. It's just that as our lives become stable, so does our character.

"The very big changes you see from early adolescence to early adulthood are greatly muted after 30, 35," personality psychologist Paul T. Costa told New York magazine's Science of Us. "There are still changes in personality after that, but they're very, very modest compared to earlier phases in the life span."

You might get a case of the pre-midlife blues.

Every decade has its crisis, and the 30s are no exception.

The quarter-life crisis -- as much a pop culture phenomenon as a psychological one -- is a predecessor to the midlife crisis that can strike anywhere from the mid-20s to the mid-30s. It tends to occur most often around age 30. Generally, this period of existential anxiety and questioning is triggered by feelings of being stuck in a job or relationship that isn't working.

"This leads to a feeling of being one thing outwardly, but feeling inwardly that you are someone else, which causes a discrepancy between your behavior and your inner sense of self," British psychologist Oliver Robinson told New Scientist.

This gives rise to a desire to change, finding an exit plan from the current situation, and rebuilding your life, Robinson explained. It can be a difficult process, but it's worth it in the end: 80 percent of young adults that Robinson interviewed looked back positively on their midlife crisis.

Real happiness is just beginning.

Once you've gotten the quarter-life crisis out of your system, it's time for life's real joy. A 2012 survey found that 70 percent of British people over age 40 said they weren't truly happy until age 33.

More than half of survey respondents said that life is more fun at 33, 42 percent said that they were more optimistic about the future at this age, and 38 percent said that they experienced less stress at age 33 than when they were younger.

“The age of 33 is enough time to have shaken off childhood naiveté and the wild scheming of teenaged years without losing the energy and enthusiasm of youth,” one of the study's authors, psychologist Donna Dawson, explained. “By this age innocence has been lost, but our sense of reality is mixed with a strong sense of hope, a ‘can do’ spirit, and a healthy belief in our own talents and abilities.”

According to another British survey, conducted by HuffPost UK and YouGov, we strike the best work-life balance at age 34, and achieve true contentment at age 38.