Age really is just a number sometimes.

And aging isn’t a linear process — our physical, cognitive and social functioning can jump back and forth between feeling younger or older.

This is known as the “flicker stage” that moves between vitality and decline — it can help us get better with age.

These “flickers” reflect how responsive the body is, as someone can feel and perform “younger” in one period and then shift toward reduced capacity thanks to illness or injury, according to Stuart Kaplan and Marcus Riley, co-authors of “Your Aging Advantage: The 7 New Stages of Aging,” out Tuesday.

“It highlights that people are not fixed in a single state of aging,” Kaplan told The Post. “Their health trajectory can be actively influenced.”

He explained that “flicker” is observable via real health markers like energy levels, mobility, immune function and cognitive performance.

“Muscle strength, cardiovascular fitness, metabolic health and even brain function can improve or decline relatively quickly depending on behavior,” he said.

“Flickers” can happen at any age, as they’re influenced by function, not just chronology.

Someone in their 30s who’s stressed, sedentary and disengaged can shift into an “older” health state, according to Kaplan, while someone chronologically older who “maintains strength, purpose and connection can operate at a much higher level of physical and cognitive function.”

“What can change with age is sensitivity and more rapid health fluctuations,” he said. “As we get older, the body often responds more quickly to both positive and negative inputs.”

And feeling older can have a negative effect on long-term health, with research showing that those with more positive outlooks on aging live an average of seven-and-a-half years longer.

“If someone ‘feels old,’ they are more likely to reduce their movement, withdraw socially and avoid challenge … which has a detrimental impact on our health,” Riley told The Post.

“Conversely, people who feel capable and engaged tend to stay physically active, maintain stronger immune responses and preserve their cognitive function,” he said.

This means that social, physical, psychological and even environmental factors play a large role in how young — or old — we feel compared to our actual age.

Some of these factors can be influenced, like increased social and physical activity to help us age backwards, while others, such as stress, may advance our “feels-like” age by a decade.

For example, inactivity can reduce muscle mass and blood flow, while exercise has the opposite effect, rebuilding strength and enhancing cognitive performance.

While not all of these factors are in our control, Riley offers up three practical ways to tip “flicker stages” towards a younger age.

“Firstly, staying in motion — physically and mentally — because as we know, the body and brain respond best when being used,” he said.

He also suggests fostering intergenerational connections to broaden perspectives and disrupt harmful stereotypes.

And the third tip is to constantly be doing or learning something new, whether starting a project or contributing in a different way to “provide energy and motivation.”

One important shift we can make is not to deny aging, but instead to embrace where we’re at and use it to age better, according to Riley.

“Aging is inevitable,” he said. “Decline is not inevitable in the way we’ve been led to believe. And that reframing really opens up our potential to age the way we want to.”