I wonder how many parents of idealistic children shuddered over their mince pies when they heard the story of 20-year-old Gerrah Selby, the attractive middle-class girl who fell in with Greg Avery, evangelical leader of the Animal Liberation Front (ALF).

Instead of going to read zoology at Edinburgh University, Selby became involved with Stop Huntingdon Animal Cruelty (Shac), the ALF campaign against animal testing at the Huntingdon Life Sciences laboratory in Cambridgeshire. Targets included British Heart Foundation charity shops because of the animal testing in cardiac research; victims were sent fake bombs and needles supposedly infected with HIV.

Selby is now awaiting sentence for blackmail and, whatever you think of the ALF and its methods, it seems a terrible waste of a life that had just begun. Her mother has said that she was always against her daughter joining the ALF, “because life had so much more to offer”.

I’m sure she tried every argument she could to tempt Gerrah away, suggesting maybe she write to her MP instead, work for the World Wide Fund for Nature, or even take a backpacking trip to find herself, but was met with the blank stare of the youthful extremist: “Democracy’s useless, Mum. None of those guys care about animals/the environment/world peace. Direct action is the only way.”

It’s a look I am familiar with as I have a 17-year-old daughter who has been a vegan since the age of 12 in a family of carnivores, a child who likes to turn the water off to save the planet while you are brushing your teeth. Her idea of a good Saturday afternoon is not browsing the rails of Top-shop but protesting against the building of Siena airport. She thinks my generation has been incredibly selfish in the way it has squandered the world’s resources: “The polar bears are dying, Mum, and it’s all your fault.” For Christmas she asked for notebooks made from recycled rubber tyres and membership of the Green party.

She won’t learn to drive because she thinks cars are wrong and she would rather stuff her shopping into her pants than accept a convenient plastic bag. She spends half an hour every night turning off all the appliances at the socket. She would rather starve than eat at McDonald’s and while she doesn't believe in God she knows that Jeremy Clarkson is the devil incarnate.

None of this is particularly easy to live with; it’s no fun having your carbon footprint ruthlessly dissected by a girl who doesn’t understand the word compromise. And yet I have to admire her unflinching stance: I can’t help feeling that this kind of passionate, unswerving, pigheaded commitment is what teenagers are for. Think of St Catherine of Siena, who had her mystical marriage with Jesus when she was about 19, or Rimbaud, who wrote all his poems before he was 20, or Esmond Romilly and Jessica Mitford running away to fight the Fascists in Spain. I would rather my daughter spent the evening arguing about the third runway at Heathrow than the relative merits of Cheryl Cole or Dannii Minogue and lining up alcopops. I approve of the fact that she buys vintage rather than Primark. I admire her for taking a stand against the “I shop therefore I am” mentality, not least because I have succumbed to it so often myself.

But when I watched the antics of the Plane Stupid group who managed to stagea protest on the runway at Stansted, I was filled with apprehension: what to me looked like a noisy and ultimately illegal protest looks to my daughter like an excellent way of spending her gap year (“I am definitely going to chain myself to things when I’m older”). However, I doubt whether my daughter, whose childhood ambition was to be an inspector of five-star hotels, would really put up with the discomfort and sheer smelliness of a Greenham Common-type protest.

For less precious teenagers there is a thin line between principles and full-monty fanaticism. What is the point at which you stop arguing the toss and start sending hoax bombs? My guess is it involves some kind of older guru like Greg Avery, or Andreas Baader in Berlin in the 1970s, or Khalid Sheikh Mohammed and the much younger 9/11 bombers: people who know that if you want to change the world through direct action then you need an army of idealistic teenagers young enough to think they know everything, but without the experience to predict the consequences of their actions. If you can channel the hormonal energy that might otherwise be spent in more carnal directions, you have the ideal foot soldiers fora crusade against the Establishment.

So I’m rather hoping my teen rebel with a cause will get past the point where she is easy fodder for charlatans such as Avery and will arrive safely in the more prosaic world of letters to The Times, Radio 5 Live phone-ins and Facebook groups. She would say that going to prison would be worth it if it meant the planet was saved, but my feeling is that such a sacrifice is pointless, because all the things she’s fighting for will come to pass in time (not a sentiment that has any appeal to a teenager whose restless months are the equivalent to adult years).

As a child of the 1980s I protested fairly feebly against racism and the poll tax and for Nelson Mandela’s release and the miners. And today we live in an officially multi-cultural society with a black American president-elect, local taxes based on property owned and Mandela is the official Global National Treasure. I am sure that when my daughter has her own two-up two-down slice of negative equity, we will be driving solar-powered cars, sending meat eaters outside to scoff their carcinogenic sausage rolls under the wind-powered patio heaters, protesting against the culling of the rampant polar bear population and regarding clips of Top Gear with the horrified awe that we now reserve for the smoking advertisements of the 1940s that said things such as: “More doctors smoke Camels than any other cigarette”.

Although my daughter has sworn not to have children because they are a drain on the world’s resources, I am willing to bet that in 25 years’ time she will be fretting over her own teenager’s desire to save the world’s endangered bovine population by patronising McDonald’s.

- Apparently we made more phone calls on Christmas Eve this year than in the whole of 1980. I wonder how many of these billions of extra phone calls have contributed to greater global understanding, economic growth or richer personal relationships?

For every call where a crisis has been averted by a politician calling his spin doctor or a marriage has been saved by a husband calling his wife to say sorry, he is going to be late, there are billions that 28 years ago would have been impossible such as, “Are you in Aisle 3? I am over by the frozen peas, if you turn round you can see me”, or “Do you think I should buy a blue cashmere sweater with a polo neck which is 30% off even if it does make me look top heavy?” or “Oh my god, I am standing behind Beyoncé and she is tiny!” or “What is the capital of Togo? One more answer and we have won the quiz”.

I wonder whether it really is good to talk or whether we have devalued the currency? I remember spending most of 1980 waiting for the phone to ring and, when the call did come, I would happily spend hours downloading everything into the ear of my beloved. Now I let the phone ring unanswered. I regard it as more of an instrument of torture and surveillance than a friend.

Perhaps if we were divested of our mobiles and put back on 1980 phone rations we might stop randomly wittering and realise that it is only good to talk sense.

India Knight is away

Times Recommends

• In the real world, the public sector must pay
• Playing politics in a land of double Dutch
• Be like the plinth people: not set in stone