The broadcaster, whose new documentary series Seven Worlds One Planet is currently playing on BBC1, made the comments in an interview with The Times.
Sir David called Thunberg 'remarkable', saying she is 'stirring the world', and adding that he's glad the climate message is reaching the masses.
He also spoke about his diet, saying he is 'not a strict vegetarian' as he sometimes eats fish, although he doesn't eat meat. Sir David admitted he hadn't given it up because of any moral virtue, but that he's 'just gone off it'.
"I can't remember the last time I had a piece of red meat," he said. "I can't tell you why. I suddenly realized I hadn't been eating red meat for a long time. I've just gone off it. I'm not claiming any moral virtue at this point - I'm just saying I don't want to eat any red meat anymore,' the much-loved naturalist explained.
"The world is changing. And with me and my family and friends there are more vegetarians or 'fishertarians' or whatever they call them than there were five years ago."