Epidemiologists have observed that the average person typically puts on 1 to 2 pounds a year from early adulthood through middle age. The CDC's numbers show that much of the increase is concentrated in the 20s, for men and women.
The average man in his 20s weighs around 185 pounds, according to the CDC. But by his 30s, he's closer to 200 pounds. The average woman's weight goes from about 162 to 170 pounds over the same period. These increases are a little smaller than the 1 to 2 pounds per year you typically hear because of different methodology: The CDC makes national estimates based on a representative subset of the population, rather than studying the same individuals over time.
There are any number of factors at work here. As people leave school, they may be less likely to participate in sports. They take on jobs and sometimes long commutes, which eat into time that could otherwise be spent exercising. Job-related time pressures make quick (and often calorie-dense) takeout meals more attractive than time-intensive home-cooked ones.
Then throw in marriage and kids for a whole different set of pressure on free time. Say you've got an hour free in the evening and you can spend it either by going to the gym or playing with your kids. Which one would you choose?
There is some good news: CDC's data suggest that your weight probably will not increase indefinitely. The rate of increase starts to slow in the 30s and 40s, and plateaus in your 50s.
After that, the average weight falls. The kids are out of the house and your career is hopefully in order, freeing up more time to take care of yourself.
But there are other factors at work at this end of the weight-age spectrum. Being overweight also carries increased risk of potentially fatal conditions, like heart disease and stroke. So across the population, some of the average post-50s weight loss may be due to people with unhealthy extra weight dying.
And as people get older, particularly into their 60s and 70s, weight loss may be a symptom of any number of age-related conditions: depression, gastrointestinal problems, cancer, etc.
One important caveat to remember is that we're talking about population-level averages here. Your individual likelihood of gaining or losing weight at any point in time is influenced by all of the things that make you a unique and special snowflake: your genetic makeup, your own workout regimen, your propensity for scarfing down Jalapeno Cheetos at lunch, etc.
Still, the average numbers give a sense of what a typical person can expect in their lifetime. So if you're in your 20s, before you ditch that exercise resolution, do the following thought experiment: Take your current weight, and add two pounds for every year until you hit age 50.
Then imagine what pants size that is.