Rowan Pelling: Girls who marry young get to repent at their leisure - so leave Kate alone

Diana, Fergie, and Princess Anne all married in their twenties, and look what happened to them

Ten or so years ago a girlfriend of mine became temporarily obsessed with Jemima Khan. "I just can't help it," she said, "it's her hair. Just look at it!" I knew what she meant. In her mid-twenties Khan's riveting cascade of glossy, wavy tresses seemed to have a serpentine life of its own. How did she manage such tousled perfection, day after day? It reminded me of Robert Browning's creepy poem "Porphyria's Lover", where the male narrator twists his beloved's hair round her neck and squeezes the life from her.

An unnaturally luscious barnet can do funny things to a person. Many men of my acquaintance still get a "funny feeling" if you mention the scene in Live and Let Die where Jane Seymour releases a waterfall of brunette locks. I mention all this because I find myself suddenly enamoured of Kate Middleton. A great part of my crush is founded on her hair. Not since La Khan first graced the pages of Hello! has there been such a salon-perfect, tumbling mane. I just want to get an ivory-backed hairbrush and stand behind her and run my fingers through her tresses and ... Whoa! Steady, Mrs Danvers.

Poor Prince William with his disappearing thatch must be utterly bewitched. And since we know the princess is the girl with the best hair, Ms Middleton would make a perfect royal bride. But is this really what we want for this enchanting, silky creature with her unaffected smile, Bambi legs and endearingly staid dress sense? Kate reminds me of my childhood Sindy doll, with her preppie wardrobe and white drawers.

Of course, the tabloids are pushing for a wedding. And "insider" reports of dubious credibility say the prince's advisers are pushing for an engagement so the relationship can be formalised before he is posted with his regiment. Those with long memories point out that it was while Charles was serving in the Navy that Camilla was swept off her feet by Andrew Parker Bowles. But what no one seems to consider is that the poor girl is still but a child. That may sound patronising, but since most of my generation (now in their late thirties) still feel like adolescents, what does that make Kate Middleton, who this week celebrates her 25th birthday?

Not that I doubt the strength of the young couple's feelings. Look at that hair! As any dog breeder could tell you, love and lavish petting adds shine and bounce to pedigree coats. But it's a peculiar plight in our flighty, hedonistic age to find true love at such a tender age. Everyone agrees it perilous to leave the quest for love too late, but nobody considers the potential pitfalls of meeting your soul mate too early.

"'Til death do us part" is an eternity at 25. Any couple who meets at university or, heaven help us, school, and finds themselves still together in their mid-twenties faces a tough decision. Should they abandon a relationship that may turn out to be the most significant one of their lives? Or, if they don't, will the relationship be haunted by a yearning for wider experience?

We cannot change the fact that nowadays most people's pursuit of love is empirical and enumerative, rather than philosophical. There's a huge temptation to think that quantities of sexual partners will teach us more than a solitary, quality encounter. And it's only by walking down that diverting, if generally illusory, path that we find quite the opposite tends to be true. I am glad for the ill-fated clinches of my youth because when I met my husband, a few months before my 26th birthday, there was such a seismic difference between that romance and any preceding it that I practically forced him to propose. Even then I remember thinking it was a slight shame he hadn't come along when I was 30, allowing room for a bit more educational misadventure.

If Kate Middleton gets engaged to Prince William she will do so in the knowledge that all the twentysomething royal brides of the previous generation, Diana, Fergie and Princess Anne, all bolted from their boring spouses as soon as they decently - or indecently - could. A man of 35 may have evolved significantly from his 25-year-old self, but it is nothing like the chasm that separates a woman in her mid-thirties from her former ingénue. Let's not forget the biology that dictates that females hit their sexual peak around 35.

Most of the women I know who married their first sweethearts absurdly young seemed to wake from a dream in their thirties. They didn't just sow wild oats, they drilled them into the soil. Look at Jemima Khan: a deferential, doe-eyed bride at 21, but a feline sex goddess in her thirties. Would we wish Kate Middleton to undergo this transformation in the stifling Windsor court? Why do we still want royal brides to be sacrificial lambs? Let the poor girl romance her prince, but retain room for manoeuvre. Personally I hope by the time she's 35 she's married to Russell Brand. Just think what larks they'd have with that ivory hairbrush...