Blood tests that seek to tell people their biological age — possibly offering a clue to their longevity or how healthy they will remain — are now going on sale.

But contrary to various recent media reports, the tests cannot specify how many months or years someone can expect to live. Some experts say the tests will not provide any useful information.

The tests measure telomeres, which are structures on the tips of chromosomes that shorten as people age. Various studies have shown that people with shorter telomeres in their white blood cells are more likely to develop illnesses like cancer, heart disease and Alzheimer’s disease, or even to die earlier. Studies in mice have suggested that extending telomeres lengthens lives.

Seizing on that, laboratories are beginning to offer tests of telomere length, setting off a new debate over what genetic tests should be offered to the public and what would be the ethical implications if the results were used by employers or others.

Some of the laboratories offering the tests emphasize that the results are merely intended to raise a warning flag.

“We see it as a kind of wake-up call for the patient and the clinician to say, ‘You know, you’re on a rapidly aging path,’ ” said Otto Schaefer, vice president for sales and marketing at SpectraCell Laboratories in Houston, which offers a test for $290.

A company in Spain, provocatively named Life Length, has begun selling a test for 500 euros ($712), that says that it can tell people their biological age, which may not correspond to their chronologic age.

Another company, Telome Health of Menlo Park, Calif., plans to begin offering a test later this year for about $200. It was co-founded by Elizabeth H. Blackburn of the University of California, San Francisco, who shared a Nobel Prize in 2009 for discoveries related to telomeres.

Calvin B. Harley, the chief scientific officer at Telome Health, said the test would be akin to a car’s dashboard signal, a “check engine light.” He compared it with a cholesterol test, but more versatile since it can predict a risk of various illnesses, not just heart attacks.

But among the critics of such tests is Carol Greider, a molecular biologist at Johns Hopkins University, who was a co-winner of the Nobel Prize with Dr. Blackburn.

Dr. Greider acknowledged that solid evidence showed that the 1 percent of people with the shortest telomeres were at an increased risk of certain diseases, particularly bone marrow failure and pulmonary fibrosis, a fatal scarring of the lungs. But outside of that 1 percent, she said, “The science really isn’t there to tell us what the consequences are of your telomere length.”

Dr. Greider said that there was great variability in telomere length. “A given telomere length can be from a 20-year-old or a 70-year-old,” she said. “You could send me a DNA sample and I couldn’t tell you how old that person is.”

Peter Lansdorp, a telomere expert at the British Columbia Cancer Agency, also had doubts. “If telomeres are short for you or me, what does it mean?” he said. Dr. Lansdorp started a company, Repeat Diagnostics, which conducts telomere testing for medical researchers only.

Recent media reports speculated on the tests and their possible implications, including ethical problems.

“You could imagine insurance companies wanting this knowledge to set rates or deny coverage,” said Dr. Jerry W. Shay, a professor of cell biology at the University of Texas Southwestern Medical Center in Dallas, who is an adviser to Life Length.

Test vendors say the speculation is running wild.

“It doesn’t mean we will tell anyone how long they will live,” said María Blasco, a co-founder of Life Length and a molecular biologist at the Spanish National Cancer Research Center in Madrid. Even if a 50-year-old has the telomere length more typical of a 70-year-old, she said, “This doesn’t mean your whole body is like a 70-year-old person’s body.”

Still, she said, “We think it can be helpful to people who are especially keen on knowing how healthy they are.”

Generally tests offered by a single laboratory do not have to be approved by the Food and Drug Administration. But the F.D.A. has been cracking down recently on some tests offered to the public, saying they may need approval. The FDA said in a statement Wednesday that it was aware of the tests, and had not come to any conclusions.

Executives at both Telome Health and Life Length say they will require a doctor to be involved in ordering the test, though SpectraCell said it allowed individuals to order the test.

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