I bought this series for my six-year-old nephew Conrad and his mum has just reported: 'Conrad REALLY loves the Bare Bum gang books! He reads them himself and is quite discerning -- we were sorting through the bookshelves only yesterday and he said he wanted to keep them for his own son to read.' That does suggest retaining them for rather a long time, but still. I'm not sure I can add more to his feedback, other than to say he's quite an advanced reader. Nonetheless, clearly the appeal stretches to a broader age range than the publisher suggests.
Recommended by one very-pleased-to-have-got-it-right aunt!