In a week when a 12-year-old girl left the country in secret, Simon Calder asks how are children able to go abroad without their parents' permission?

What's the airline industry's definition of a child?

That depends. There's no set rule about how old a passenger has to be before they can travel on their own, but British Airways is fairly typical of traditional airlines: anyone 12 or older, who anyway has to pay the adult fare, may fly solo. No-frills airlines - and the Government, when it's taxing passengers - deem anyone over two years old be an adult, but they usually impose a minimum age for solo travellers; for easyJet it is 14, for Ryanair 12.

Below this age, children can often be sent as "unaccompanied minors", a service for which British Airways is renowned - but for which it is also increasing fees from 1 August. A nominated adult delivers the child to the airport. The child is looked after by the airline to the final destination, when he or she is collected by another nominated adult.

If you try this system in America - where tens of thousands of children fly each weekend from one estranged parent to another - make sure the person who you entrust to meet the child is reliable. Southwest Airlines warns that its staff cannot look after unaccompanied minors overnight, and that they will be turned over to the police.

What does the law say about young people flying abroad without their parents' consent?

Not much. There are no aviation regulations on the subject. Airlines feel their responsibility extends simply to getting people safely from A to B. Even if there were rules, it would be tricky for the Government to enforce. The Home Office took a decision five years ago to remove passport controls on departure from the UK. And there is no easy means of checking whether or not a child is travelling with the knowledge and permission of a parent.

But airlines check passports - can't they ascertain the relationship at that stage?

Airlines reconcile tickets with passports - making sure that passengers are who they say they are - for commercial and security reasons. They don't want a "grey market" in secondhand tickets, and they prefer to know exactly who is on their planes. Check-in staff are not looking for evidence of the suitability of an adult accompanying a child.

Why not?

Partly from expediency. In today's fluid and fragmented society, it would be extremely difficult to ensure that adults accompanying children are blood relatives or legal guardians. Britain's holiday airports will be heaving with people today, and a significant proportion of the children being checked in will have different surnames from the adults accompanying them.

So long as a child is not visibly upset, there is no obvious reason for staff to question the relationship. And since air travel is a high-stress activity for a lot of participants - passengers and staff alike - even distressed children might be allowed to board a flight with an adult who is not related to them.

How do they do things abroad?

Plenty of countries try to control the movement of young people. In France, anyone under 18 who wants to leave the country using a national ID card has to get a document known as a quittance du territoire from the local town hall. Portugal actually refuses entry to foreign youngsters not accompanied by a parent or legal guardian.

Elsewhere in the world it's even tougher. If you plan to visit Mexico, for example, try to make sure that both parents go along: if the child is travelling with only one parent, the other parent has to provide a legally certified document consenting to the journey - and the same applies if the surnames on passports don't tally.

In loco parentis on the trains?

Not a chance. Eurostar, which runs trains through the Channel Tunnel between London and Paris, has no mechanism for reconciling the name on the ticket with the bearer's passport. Indeed, there is a flourishing market in buying and selling unwanted tickets. The train operator appears to feel that it is the parents' responsibility to ensure that their child is not travelling with a stranger.

Is there anything that Britain could do to avoid cases of child abduction?

Yes: go back to the practice of checking the passports of people leaving the country, which prevailed for decades until it was abandoned to save money in 1998. Five years on, the technology has improved to the stage where it would be possible to find out exactly who's leaving the country, without dramatically increasing queues.

Whatever your views on the desirability of our "fortress Britain" policy, it can't work properly if there's no way of telling who's in the country. Around 60 million people leave the country each year - a number equivalent to the entire population of Britain - yet the Government has no idea who they are.

The Home Affairs Select Committee called for much greater use of available technology in a report two years ago, and also argued for a single border control authority as has been established in the US, but so far there have been no significant changes.

Checking passports of people departing from Britain will be seen by some as extending the reach of Big Brother. Others would maintain that, post-September 11, keeping tabs on the movement of people is an essential adjunct for national security. The Americans certainly think so; the CIA has the right to trawl airlines' reservations systems to find out who is flying to, from or merely transiting the US.