Think before you tweet.

The stillborn political career of 17-year-old Paris Brown is a parable for our time. One moment she is about to become England's first youth police and crime commissioner, beaming and blushing beneath her bouffant hair as she pledges to change the image of young people in Kent and act as a messenger between her generation and the police. The next she is crying off her mascara on television after a Sunday newspaper published selected extracts from her Twitter account in which she refers to gay men as "fags", immigrants as "illegals" and travellers as "pikeys". On Tuesday she stood down from the £15,000 a year job.

She produced the standard excuses: that she was young, that she was "venting" and that the remarks had been "taken out of context". Her sponsor, Kent PCC Ann Barnes, even suggested: "It would be impossible to find a young person who had not made a silly, offensive or perhaps even deeply offensive comment during their short lifetime."

Really? Surely it's not OK to make insulting remarks about someone's race or sexuality simply because you are an angry young woman. And if this is the wrong context, what is the right context for racism and homophobia? How could she have represented all young people, including presumably those who are black and/or gay, after such a breach of trust?

Ms Brown also claimed: "When I tweet, I do often exaggerate." That's much nearer the mark, though still does not excuse her slurs and innuendoes. I reviewed teenage literature for this newspaper for nearly two decades. Few teen titles transfer to the adult market for the simple reason that they are so emotionally overblown. As I discovered when I had three of my own, that's because teenagers are like that. In meteorological terms, they are capable of going from flat calm to violent storm force 11 and back again in five minutes flat.

What's changed since I threw the odd teen tantrum (usually about the length of my school skirt) is that the new social media have torn down the distinction between our kids' private and public lives, with potentially devastating results. Some kids, particularly the offspring of the rich and famous, are virtually turning themselves into brands. But teenagers seem more sophisticated than they are.

Ann Barnes opined: "I'm sure many people today would not have the jobs they are in if their thoughts in their teenage years were scrutinised." What planet is she on? Last year the website CareerBuilder found that more than one-third of employers now pre-select job applicants after looking at social media sites and 13% of applicants are rejected for making racist comments. Unless they pay close attention to privacy settings, our kids have the equivalent of a CV before they even leave school. Former Google CEO Eric Schmidt once suggested, only half in jest, that teenagers might like to change their names when they come of age to distance themselves from their dodgy online past.

We're dealing here with a contradiction. There is a confessional quality to Twitter and Facebook that encourages users to share secrets and random thoughts, without concern for the consequences. Yet those tweets and posts form a permanent record. They are the equivalent of shouting through a megaphone or splashing your message across a billboard. Sites like Simple Wash, My Permissions and Tweet Eraser may help you clean your profile but there's no guarantee that you'll reach every cache. It's virtually impossible to erase your internet track entirely.

We can rail against Google and Facebook for solving technical problems without considering ethics but cannot uninvent them. Maybe in time we will learn to live with them. Perhaps employers will allow people to outgrow their youthful indiscretions. But until then we need to teach our teenagers self-censorship.

There is a small sad irony in the tale of Paris Brown. She's learned a lot in a week. In fact, today perhaps she's closer to being the sort of young woman required to be England's first youth police and crime commissioner.