Chronological age refers to the actual amount of time a person has been alive. The number of days, months, or years a person has been alive does not change, regardless of how healthy a lifestyle—even one filled with great exercise and nutrition habits—they are living.

Everyone ages at a different rate. Some people seem to age very rapidly, while others experience aging at a much more gradual pace. We've all experienced meeting someone for the first time who appears to be much younger—or older—than they really are.

Chronological vs. Biological Age

As humans, we have two different ages—chronological and biological. Chronological age is the number of years a person has been alive, while biological age refers to how old a person seems.

Biological age, also referred to as physiological age, takes many lifestyle factors into consideration, including diet, exercise and sleeping habits, to name a few.

How we age is beyond our control. It's primarily influenced by genetics, but research sheds light on how aging can be impacted by external factors, including diet, exercise, stress, and smoking.

Many gerontologists believe chronological age to be an incomplete figure because it doesn't take these external factors into consideration.

How Biological Age Is Determined

Research suggests that telomeres and DNA methylation play big parts in the aging process.

Telomeres

Telomeres are the nucleotides on the ends of chromosomes. They keep the ends of chromosomes from deteriorating and fusing with a nearby chromosome. Essentially, telomeres dictate how quickly cells age and die.1

Scientists have discovered that the higher a person's chronological age, the shorter their telomeres. One study found that people with shorter telomeres were more likely to have an early death or develop a disease or neurodegenerative disorder.2

Another study suggests that maintaining a healthy lifestyle can actually reverse aging by lengthening telomeres, good news for our age-obsessed culture.3

DNA Methylation

Scientists are also using DNA methylation to determine biological age. Cells use DNA methylation to control gene expression. In other words, DNA methylation turns genes off. Although the exact purpose of methylation is unknown, it's vital to embryonic development, genomic imprinting, chromosome stability, and more.

One study sought to discover whether DNA methylation is an accurate way of predicting age by gathering 8,000 samples of 51 different tissues and cells.4 Most of the tissue and cell samples studied had the same chronological and biological ages. Some didn't.

The study found that certain parts of the body age faster than others. For example, breast tissue is some of the oldest tissue in the body. The research found that even healthy breast tissue can be as much as three years older than the rest of a woman's body. If healthy breast tissue exists near cancerous tissue, it's an average of 12 years older.