There is a moment in a young princess's life when she finds her true place in the nation's heart. For Beatrice it happened this week when media open season was declared upon her avoirdupois. For years, press snaps of her and sister Eugenie, bursting lustily from the slightly deranged matching garments selected by their mother, have been captioned with restraint appropriate to vulnerable, growing girls. But, heck, she's 19 now, old enough to handle “give the girl a sarong!” bitchery about her bikinied bum. And if she can't, she'd better count Weightwatcher points with Fergie or succumb to a fashionable food disorder like her late aunt.

So the cast of young royals is fully assembled. Positions of wild, dashing, sporty and dull are covered respectively by Harry, William, Zara and Peter. And now Beatrice has stepped up to comedy royal perhaps, or more likely Duchess of Pork II. Eugenie, still only 18 and a bit of a cipher, has at least been stretched and airbrushed into lushness on the cover of Tatler. She should be aware the parts of flighty or drug-dependent are as yet uncast.

It is strange to look back upon the last era when a shiny batch of freshly minted young royals bestrode the tabloids. In the mid-80s such standards of stiff formality were maintained that when Fergie and Di goosed a man with an umbrella at Ascot, it was believed to augur the fall of the House of Windsor. Which perhaps it did, or at least a bracing wind of glasnost through The Firm.

Now here we are with Zara wielding a whip for a lads' magazine, getting so blasted she fell backwards off her chair, cracking her head on a table, cohabiting - in her mother's backyard! - with a great lunking commoner. Charles and Diana's post-wedding balcony kiss was iconic, a never-seen-before royal intimacy. These days, with Harry's tongue rarely out of Chelsey's ear, the first royal internet sex tape cannot be far away.

But what is fascinating from a constitutional perspective, is that even princely porn, even allegations of a royal snorting cocaine while pleasuring his manservant, cannot shake the monarchy. Indeed, today the tumbrils are strangely distant. And this is not due entirely to the everyman inclinations of William and Harry, eschewing valets, crapping alongside comrades in Afghan foxholes or visiting amputees. It is because young royals have merged into a broader celebrity culture and as such are little more taken to task for their morals or excesses than, say, David Beckham or Lindsay Lohan.

The royals are merely hereditary celebrities. Who cares that William flew his military chopper to a stag do or Eugenie and Beatrice bask on tropical beaches accompanied by a £250,000 tax-funded security detail. We have lately grown accustomed to letting the super-rich live unchallenged according to private rules: the divine right of bling. And, unlike non-doms, at least the Queen pays tax.

When I was a teenager the House of Lords seemed the very embodiment of privilege and injustice. But, I was told, to reform it would undermine the hereditary principle, the whole monarchy, root and branch would wither and die. And yet the opposite has occurred. Now that (most) peers from the shires no longer legislate on our affairs, the sting is gone. Monarchy thrives on a happy island of quaint tradition, cut off from broader political life.

It may be undemocratic and ever so unfair that a young man gets a bunch of palaces and the title king purely through dint of birth. But young people now live with the hope that equally random factors - reality TV stardom, lottery wins, sleeping with a Manchester United footballer - will raise them above the herd.

And the line between royals and celebrity has blurred. Madonna and Sting live as feudal barons in grand estates; Zara Phillips and Helen Taylor advertise Armani or Land Rover like common footballers. Celebrity has raised the glamour bar for royalty. There is no longer any pressure to downscale, to shed palaces, to become modest-living Scandinavian cycling monarchs: that would be as tiresome as being ruled by Chris Martin and Gwyneth Paltrow.

All we require is that our young royals be eager and broadly attractive characters in our national soap, permit us regular insights into their hearts and wardrobes. That the young royals are, of course, upper class is no impediment to public affection. The democratisation of culture has seeped both up and down, making a star of a talent-free low-life such as Jade Goody but also ending prejudice against those born properly posh. An Eton education and cut-glass elocution - as Wills, David and Boris are discovering - is no barrier to mass appeal. The one baseline measure is being a good bloke: clubbable, decent, liberal, above all not “up himself”.

Which is the Princes of Wales's problem. At his age, the status of national treasure might be expected, although admittedly it is easier to attain that as a woman when you just need to start resembling the nation's gran. But in trying to fill his life with meaning, Prince Charles has accumulated a surfeit of opinions. And these, rather than that he is old and odd-looking, have excluded him from the warm embrace of celebrity culture. Opinions are annoying. That he turned out to be right - as on organic farming - is more annoying still.

The young royals share with their generation a blithe, apolitical blandness and, apart from vague green sentiments, have little to say, no pressing agenda for the world. The monarchy's sole remaining problem is that the throne will not pass from the dear Queen to the good bloke, but endure an irritating, probably comedic interregnum.

There was outcry recently that Gordon Brown had decided to abandon reform of the Act of Settlement which forbids royalty to marry or become a Catholic and enshrines good old sexist primogeniture. (Justice for princesses! Now there's a campaign with mass appeal.) Indeed, the Canadian bride of Peter Phillips has renounced her faith to save his place as 11th in line to the throne. Does Peter really think he has an outside chance of the crown? Or did his wife convert willingly, weighing up that if her fiancé lost his royal lineage she was just marrying an average bloke who works for the Royal Bank of Scotland. But if the Act of Settlement is amended, changes need to be far more sweeping, better to reflect the new celebrity nature of monarchy: “Vote for king by texting 0801 if you want Wills, 0802 for Harry...”

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