Molecular change in the body doesn't move at the same pace every year. A Stanford University study published in Nature Aging in 2024 showed that thousands of molecules in the blood and the microbiome shift together around ages 44 and 60. Other teams have found ages such as 34, 50, 60 and 78. The shared conclusion is this: change arrives in waves, and the period around age 60 recurs in many of the studies. The exact age of the waves, though, moves from study to study, and their causes are still unknown.
The finding that headlines boiled down to "we suddenly age at 44" is in fact a narrower and more cautious claim than that. Below we go through, in order, what the studies measured, what they didn't measure, and why the numbers don't agree with one another.
The team of Xiaotao Shen and Michael Snyder followed 108 people aged 25–75 living in California. While they were healthy, participants gave blood, stool, skin, oral and nasal samples every 3–6 months. A total of 5,405 samples were collected, and from them 135,239 distinct biological features were measured, covering gene expression, proteins, metabolites, cytokines, lipids and microbiome data. The median follow-up was 1.7 years and the longest was 6.8 years.
The first finding was a surprise: only 6.6% of the molecules examined changed in a straight line with age. By contrast, 81% differed in at least one age bracket when compared with the 25–40 age group. The researchers then slid a window along the age axis to see at which ages the most molecules were changing. Two peaks emerged. The paper's abstract gives them as roughly ages 44 and 60; in the body of the text the same peaks also appear as "around 45 and 65" and "around 40 and 60". In other words, 44 is less a sharp threshold than an estimate pointing to the mid-forties.
Some groups are common to both waves: pathways linked to cardiovascular disease (coagulation, the complement system), proteins involved in skin and muscle structure, and caffeine metabolism. A separate analysis showed blood urea nitrogen and blood glucose rising faster after age 60; the authors interpret this in terms of kidney function and type 2 diabetes risk. The immune shift in the age-60 wave fits the same picture as the chronic low-grade inflammation we describe in our inflammaging article.
One detail shouldn't be missed here. The measurements were made on blood and microbiome swabs. The "skin and muscle" finding is an association suggested by molecules in the blood; no tissue was taken from anyone's muscle or skin. The authors list this as a limitation in the paper themselves.
This is the first explanation that comes to mind, because menopause mostly happens between ages 45 and 55 and 51.9% of the participants were women. So the team clustered the data for women and men separately. Similar patterns emerged in both sexes. The authors' conclusion is that the midlife turning point cannot be attributed to menopause alone and is a phenomenon shared by both sexes.
That doesn't mean menopause has no effect. A study published in Nature Aging in 2026 examined 1,112 tissue images and 659 RNA samples from seven female reproductive organs, taken from donors aged 20–70: while the ovary aged gradually, the uterus went through an abrupt molecular and structural change around menopause. For women, menopause is a breakpoint in its own right; we cover the topic in our article on menopause and hormone therapy. The cause of the mid-forties wave, which is also seen in men, remains an open question.
The Stanford finding doesn't stand alone, but it doesn't match the other studies one for one either.
The numbers failing to line up doesn't show that the findings have fallen apart; it shows that what is measured, and how, determines the result. When the Stanford team compared its own protein data with the 2019 study, it saw that the two measurement platforms (mass spectrometry and SomaScan) captured largely different proteins: there were 302 on one side and 1,305 on the other, and only 75 were shared. The authors attribute the roughly ten-year gap in the first wave to this. As for the third wave, they couldn't see it at all, because their oldest participants were 75. Nor would blood plasma be expected to follow the same timetable as vascular or liver tissue; as we explain in our organ age article, organs age at different rates.
Two conclusions from this picture are the most solid. First, the period around age 60 recurs in four of the five studies. Second, the early wave wanders between 34 and 50; "44" is an estimate from inside that range and shouldn't be read as a date to mark on the calendar.
Let's be honest first: none of these studies tested an intervention. There is no data that would let anyone say "do this and the wave at 44 will be milder". What follows are the places where the areas the findings point to overlap with health advice that is already well established.
If you're curious about your own pace of aging, you can read about measurement methods that are independent of chronological age in our article on epigenetic clocks. Just like these waves, they are tools that are meaningful at the group level and need to be interpreted with care for any single person.
Nobody is describing a sudden change that happens on a birthday. The Stanford study found that the levels of thousands of molecules in the blood and the microbiome shift together during the periods around ages 44 and 60. That is a group average drawn from comparing 108 people of different ages; it was not shown that individual people go through a jump at these ages. What was found is a molecular pattern. The aging a person feels or sees in the mirror was not measured.
Menopause alone does not explain it. When the researchers analyzed the data for women and men separately, they saw similar patterns in both sexes and concluded that the midlife turning point cannot be put down to menopause alone. The actual cause of the wave is unknown. The authors themselves note that lifestyle differences, such as alcohol and caffeine intake or physical activity, may have influenced the result.
The studies measure different things with different methods. The 2019 plasma protein study pointed to ages 34, 60 and 78, the Stanford study to ages 44 and 60, and the 2025 tissue proteome study to around age 50. As the molecules measured, the instruments used, the age range of the participants and the analysis window change, the peak moves too. What they share is that the period around age 60 shows up again in many of the studies, and that in none of them does the change follow a straight line.
In the Stanford study, molecules related to alcohol metabolism changed markedly in the 40s, and those related to caffeine metabolism changed in both the 40s and the 60s. But the researchers did not know how much alcohol and coffee the participants consumed. The change could come from the body's capacity to process these substances, or from consumption habits shifting at these ages. The study cannot tell the two apart.
None of these studies tested an intervention, so no particular habit has been shown to soften the waves. Even so, the areas the findings point to line up with advice that is already well established: cutting back on alcohol, preserving muscle mass with resistance exercise, and keeping up with blood pressure, blood lipid and blood sugar checks. Your doctor decides which screening you need at which age, based on your personal risks.
"More than one study has seen aging move forward in waves. Exactly what age the waves arrive at, and how a person experiences them in their own life, has still not been measured."
— Vivelong Longevity Science Team