The 79-year-old actress said people should make "little changes now to help them later in life".
This "doesn't have to include joining a gym" but could include short walks or yoga, the Oscar winner said.
It comes as Age UK said there is a "major concern" about people's physical health and wellbeing as they age.
The charity has called on people aged 50 to 65 to make exercise a "key part" of their routine as part of its latest campaign.
England's chief medical officer Professor Sir Chris Whitty said keeping fit in midlife is one of the "main preventive strategies" for staying healthy in older age.
Backing the new campaign, Dame Helen, an Age UK ambassador, said: "Whatever you may think about ageing, it's something we must all try to embrace rather than fear.
"It's no secret that for many, getting older will come with its complications - specifically there will likely be aches and pains along the way, getting out and about and living as independently as we're used to in our younger years may not be as easy to do.
"This is why I'm supporting Age UK's Act Now, Age Better campaign, they're encouraging everyone in their 50s to make little changes now to help them later on in life.
"It doesn't have to be joining a gym; it can just be small changes like taking a short walk or yoga, which I still enjoy.
"It's never too late to start doing something, so why not give something a go while you're younger?"
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