The book blurbs say that Nancy Drew is the “best-known and most loved girl detective ever”, but here is a mystery that even golden-haired Nancy might not be able to solve. Why is she getting younger and sillier by the decade?

The first Nancy Drew book came out of the American fiction factory of Edward Stratemeyer and appeared in 1930. The ghostwriters have been busy ever since, turning out scores of titles while the identity of author “Carolyn Keene” remains a coy secret. Nancy has managed to advance officially only two years - from 16 to 18 - but her ability to hold age at bay is hardly surprising given her all-round prowess.

She manoeuvres boats and planes and drives with the greatest of ease. She explores the darkest basements and tunnels and spies from the most frightening of cupboards. In her free time she picks up golf championships and tennis trophies, or creates her very own version of Morse Code for tap-dancers.

The original series fulfilled a sort of Blyton-esque function in that girls at the age of 10 could be seen tackling 200 or so pages with real gusto. I was one of those girls and I am in good company: in the past 67 years more than 80 million copies of Nancy Drew titles have been bought.

Inevitably, Nancy has been rediscovered, updated and plunged into romantic intrigue, scandal and big business blackmail (or so says the blurb on the back of one such update). Now there are half a dozen series in circulation, aimed at ages seven and up. But it is hard to recognise our heroine in the new Notebook series, aimed at younger readers. Whatever happened to the poised, clever and quick championship golfer and Morse-code tap-dancer from The Clue of the Tapping Heels (circa 1939)?

The answer, as revealed in The Lost Locket and The Slumber Party Secret, is rather depressing. As an eight-year-old, who has to rely on grown-ups, Nancy is far less interesting. She lives as per normal with her famous father and their housekeeper; she is still friendly and inquisitive; her best friends are still Bess and George.

But there is no sign of the remarkable young woman to come. The girl who grew up to fly planes, read Middle English and collect trophies effortlessly - with nary a boy by her side - is reduced in these ultra-juvenile mysteries to figuring out which of her classmates eats peanut butter, mustard, ketchup and relish sandwiches.

The younger Nancy spends a lot of time trying to get Bess and George to stop fighting. The child who we expect to grow into action girl seems, in these books, to be set for a career as a relationship counsellor. She is also far too girlie for her own good. It is not possible that the real Nancy Drew would be as interested in rainbow hair-clips as she professes to be in The Slumber Party Secret.

If girls of nine and 10 can read the real thing - and they can, they can - it seems a shame to dilute the Drew factor so very much for marginally younger readers. It’s hard to believe that this pale imitation could mature into the Nancy we know and admire.