You must be very clean," Marie's friend said on the phone one Sunday evening. "You've been in the shower most of the weekend."

It was time, obviously, for Marie's 11-year-old son to learn a few new stock replies for answering the phone when his parents are out.

There comes a time in every family's life when the youngest - or perhaps only - child seems too big to be parcelled up and left with relatives, friends, neighbours, babysitters or creches when his or her parents are out. But are children of 11 and 12, even 13 - the awkward "tween" age years - too young to stay home alone?

When children are small, most parents wouldn't consider leaving them by themselves in the house, even to dash to the local shop. But the temptation to do so grows along with the children. If both parents work elsewhere, or if you're a single parent working outside the home, with childcare arrangements based around school hours, you're in trouble when anything unplanned crops up. When children are younger, you have no choice when sickness, rogue school closures, uncoordinated mid-term breaks or a change in your work schedule creates problems: you find someone to mind little Saoirse or Sorcha or do it yourself. If you have to go out, however briefly, at night, you find a sitter or take the children with you.

But your 11-year-old seems sensible. Many 12-year-olds babysit small children themselves. Surely it's okay to leave them home alone?

No, says Norah Gibbons, childcare director of Barnardos, sympathetically but unequivocally. "At 11 and 12, they're too young to be left alone for any length of time." And that's in the daytime: leaving them alone at night-time is "a big no". And whatever about leaving an 11-year-old for 15 minutes while you go to the shop, leaving them for half or all of a day isn't on, she says.

There is no law telling parents when it is okay to leave children by themselves, but parents do have a legal responsibility to "act reasonably in relation to their children". And parents can be and have been prosecuted for neglect, a sobering thought when you're considering all this.

Of course situations vary, children differ, and "it is impossible to define exactly at what age you can give what responsibility to a child". In practice, says Gibbons, you have to consider not whether your children will manage in normal circumstances, but how they would cope if something goes wrong?

"Despite how sensible many 13-or 14-year-olds might be, they can panic and react in an unexpected way. No one knows, for example, when a fire might break out."

Children might use the cooker, even if told not to, she points out, and could do something as simple as scald themselves making a cup of tea.. "Would they know what to do then?"

She agrees that a lot of parents do take a calculated risk, and understands how it can happen. Apart, of course, from it being convenient for parents not to have to find a minder, many 11- and 12-year-olds object to being babysat, and are eager to prove how grownup they are.

One 11-year-old explained recently to his mother how he might cope if he thought someone was trying to break into the house and he was by himself: "I'd get behind the TV set, and I'd pick up the blackboard, and I'd hit him over the head - that'd be sure to stop him. And first I'd ring 911 [the American emergency number!]." Proof, if proof were needed, that her son wasn't as grown-up as she'd thought. Another family had a scare when their 12-year-old son, recuperating at home after a mild illness, was left alone for a morning. People claiming to be repairmen called to the door and asked the child to check if the TV was working. He had the wit to close the door before answering them, but later rang his mother when he noticed the men going from house to house in his quiet cul-de-sac.

As it turned out, the repairmen were genuine - but how would the child have coped if they had been burglars?

Parents shouldn't be swayed by an 11- or 12-year-old child's willingness to stay home alone, says Gibbons. "They have to explain to the child that parents are responsible for their children's welfare." When you do start leaving children alone for a couple of hours or more at a time, you should be very clear about how they should behave, says Gibbons. She recommends that they shouldn't answer the door or the phone - this is much simpler than having them invent all sorts of reasons why their mother or father can't come to the phone. You should make sure there is a neighbour nearby to whom they can turn for help if they need it - and you should make sure that that neighbour is going to be in, and is willing to help.

The child should have a telephone number where one or both parents can definitely be contacted. It's probably also a good idea to display local emergency numbers prominently in your house - many Sky-addicted children do half-think they live in the US.

Age alone isn't the only factor in deciding how much responsibility to give a child, says Gibbons. "You have to know your own child: some 13-year-olds are very sensible, some are giddy."

This issue obviously also has implications for babysitting arrangements - both parents of babysitters and parents of small children should be cautious about letting 13- and 14-year-olds babysit.

"You have to ask `Do I really expect a child to act responsibly, to know what to do in the event of something going wrong? What if one of the small children gets very sick, for example,' " Gibbons says.

Ninety-nine per cent of the time nothing will happen when children from 11 to 14 are left home alone. But they're not all Macaulay Culkin (thank God), and lack the vicious cunning that saw him survive being left by himself through two movies. In the real world, children under 14 can be very vulnerable.