'For me, the films have always been about a loss of innocence,' said Radcliffe. 'When Harry came into this world, it was all just amazing and brilliant and kind of pure. But as the films have gone on, that's totally disintegrated, and he's realising that the wizarding world has just as many, if not more, challenges than the world he grew up in before.'

Just as their characters are growing up fast in the film, so Radcliffe, Watson and Grint are charging into adulthood away from the set.

They were aged 11, 10 and 12 respectively when they were chosen for the first Potter film, which hit cinema screens in 2001.

Radcliffe, who turns 20 on July 23, famously appeared naked on stage in New York and London productions of 'Equus' to blow away his straitlaced image.

Watson, now 19, radiates understated English chic modelling for fashion house Burberry, her way of blowing away the frumpy jumpers and backcombed hair she wears on set as the academically brilliant Hermione.

'When I was little, I didn't understand that other kids thought I actually was Hermione, really geeky. It was devastating. I thought no one would ever fancy me,' Watson said in a recent interview with The Times.

Grint, meanwhile, has had to contend with a mild bout of swine flu but is expected to be fit to attend the premiere.

Stuart Kemp, London correspondent of Hollywood Reporter magazine, said the Potter films had become one of the most successful film franchises in history by evolving with the books' core audience.

'They are getting more grown-up and sinister as the fans grow up with them, and they have become progressively darker.

'When the series does end, it will be a huge loss to the British film industry, both in terms of box office and jobs. They have employed a lot of people and they will leave a sizeable hole.' -- AFP