Don’t waste money on tests that promise to tell you how fast you’re aging, researchers say. A new study suggests that we don’t know enough about the body’s aging process for them to be useful.

Plenty of expensive tests claim that looking at your genes, a drop of blood, or other physical indicators can tell you your “biological age” — or how fast your body is physically breaking down, regardless of what the calendar says. There is evidence that these markers can give you a rough idea of how healthy your body is. In a study published this week in the American Journal of Epidemiology, scientists looked at the health data of nearly 1,000 New Zealanders, collected from birth to age 38. They analyzed 11 biological markers — including blood markers and gene changes — in the same people from ages 26 to 38. But none of these markers agreed on how fast someone was aging. And many of them weren’t good at predicting things like physical and cognitive decline, as measured by balance, grip, cognitive tests, and how old their faces looked.

People do age at different rates, but there’s no agreement on which measures really tell you the truth, the study shows. Also, aging happens at different rates in different places, according to the researchers, so you might get one answer from blood markers and another from how good your balance is.

One of the most well-hyped measures of biological age involves measuring the length of the ends of DNA, called telomeres. Telomeres are like little caps on the ends of DNA strands that keep the genetic material from fraying, and they become shorter as we grow older. (Three scientists received the 2009 Nobel Prize in Medicine for discovering this fact about telomeres.) But the researchers found that telomere length didn’t predict cognitive or physical decline, though it had some correlation with how old people’s faces looked (as judged by others).

The team also looked at changes at the specific places in the DNA to look for patterns that correlate with aging. The scientists measured three different DNA patterns when the participants were 26, and then again when they were 38. These patterns did show that 12 years had gone by, but, again, it didn’t predict cognitive or physical decline.

Finally, the team used algorithms to see if there was a correlation between physiological markers like lung function and heart function and biological age. They found stronger results than with telomeres or the DNA patterns, but it still wasn’t robust. So, the study concludes, those aging tests are almost certainly premature. All these markers are probably measuring something — but until we have better data, it’s better to save your $300.