Before we get started, we'd like to mention that anyone with a 30th birthday coming up might want to skip this one (you've been warned). Research on happiness and age, published this week in the journal Social Psychological and Personality Science, found that a major shift has occurred in the last five or so years: The number of people over 30 claiming to be "very happy" is the lowest it's been since the study began in the 1970s. Yikes.The study took place over 40 years — between 1972 and 2014 — and used four representative samples of more than one million Americans. Researchers write that this was an investigation into "subjective well-being. So their main method of measuring happiness was a simple question, posed to each participant, along the lines of: "Taking all things together, how would you say you're feeling these days — would you say you’re very happy, pretty happy, or not too happy?"Let's start with the good news: Teens and young adults (between 18 and 29 years old) are only getting happier. In the '70s, only 19% of teens claimed to be "very happy," compared to 23% in the 2010s, while "very happy" young adults went from 28% to 30%. Now for the less-than-great news: The portion of people over 30 who said they were "very happy" fell from 38% in the '70s to 32% by the end of the study.Of course, those shifts are all quite small individually. But one other thing stands out:Could this be the end of the line? Is 30 now the cutoff age for feeling truly happy? Probably not. For one thing, we find and experience happiness in different ways depending on our age. Could it be that those thirtysomethings who feel less than "very happy" are just having a bit of a Jack & Diane moment? That's one suggestion the study makes: "Increasingly unrealistic expectations for educational attainment, jobs, material goods, and relationships may feel good for adolescents but be disappointing for mature adults who cannot attain these goals."So all you 29-year-olds, keep your chins up and have hope. There are plenty of ways to prepare for your 30s — and even a thing or two to look forward to. In fact, you might end up having such a lovely time that you forget to worry about being happy at all.