What it means to be physically fit is a 'how long is a piece of string?' question, says celebrity trainer Luke Istomin, who has worked with Hugh Jackman and Nicole Kidman.
"There are a billion variables, but fitness could be defined as the ability to undertake your daily tasks without being out of breath or without exertion," Istomin says.
Of course, those daily 'tasks' will vary dramatically from person to person and from your twenties to your sixties.
But wherever you are at, there are tools to help you feel better in your body and to feel fit for your life.
We've created this interactive so that you can choose your workout according to your age and fitness level.
"The benefits of a consistent training regime range from mental health improvements and cognitive function, to joint and tissue strengthening and injury prevention and … feelings of self empowerment and fulfillment," says Istomin, a trainer of 20 years and co-founder of F45.
"There is no better time than to start training than right now."