The first time parents do it, they take precautions. Some hide the chip pan, just in case. Some get a neighbour to park in the driveway. Sooner or later - and very often, on a Bank Holiday weekend like the one just past - most of us have to answer that tough question: when is it safe to leave a child home alone? For Maeve, the issue arose a few summers ago, when her daughter was 17. She and her husband wanted to go on a week-long holiday to the country with their three younger children. Their daughter was working, an excellent reason to refuse to come along. And so they agreed to let her stay at home, as long as a sensible friend stayed with her. And despite their most lurid fears - parties where gatecrashers strip the tiles from the roof, break-ins, fires, boyfriends staying over - it passed off without incident.

Okay, there was one morning when her daughter stayed out until 5 a.m., which probably wouldn't have happened if her parents had been at home. But on the whole, she passed the responsible teenager test with flying colours.

Holidays are often the trigger for a decision most families eventually have to face - unless the parents never leave home. Or sometimes it's a family emergency, like having to visit a sick parent in another part of the country. No matter how sensible your teenager, the first time you do it, you worry.

Deirdre left her two eldest children, both girls, alone overnight for the first time when they were 15 and 16. "We had to go to a wedding in the country. I stayed awake all night in the guesthouse, and we were back before they got out of bed."

The next time, she and husband left the two girls alone for two weekend nights when they had to go and visit Deirdre's elderly parents in the country. She trusted her daughters to be sensible and they were. Deirdre's biggest fear was a breakin, so she asked her neighbour to park his second car in her driveway and told the girls to leave the lights on downstairs all night. And she emptied and hid the chip pan, just in case they might put it on and forget about it.

By the time they were 17 or 18, Deirdre trusted her girls enough to say they could have a few friends around.

Now her teenage son, who is nearly 16, wonders when he will be allowed the privilege of being left on his own - but Deirdre thinks she might never get to that point with him, because he's just not sensible enough. "He's exactly the child who'd have the wild party."

Sean Mitchell, president of the National Parents' Council (Post-Primary), reckons the dreaded party is possibly the biggest concern many parents would have about leaving teenagers in charge of a house. "I'd be aware of groups of youngsters availing of a `free gaff' most weekends," he says - which indicates that there are parents too careless or too innocent to know that their child might well have a party of some sort if they leave them unsupervised.

Mitchell thinks it's not carelessness: "Most parents wouldn't be happy if they know that 10 or 12 or more teenagers were gathering in their home without their knowledge, and the damage they might do could be the secondary consideration. Most parents are blissfully unaware of what can happen." The "free house", while essential to the plot of many a teen movie, is something that strikes fear into many a middle-aged heart, and with good reason. It's not that teenagers won't drink, take drugs or engage in unapproved sexual activity without a free house, but it sure makes all of these easier.

Horror stories about the damage an out-of-control teenage party can do - neighbours' cars dented, furniture broken, gardens uprooted are features of the milder tales - would make most of us stop and think. And even if your children are responsible beyond their years, they may not be able to resist peer pressure to take advantage of the free house. "A teenager might start the evening with the best of intentions, to have just a few beers or a chat with a few friends. But there would be significant peer pressure on them to have an open house, and not many of them would be able to resist it before the age of 18 or 19."

Fionnuala Kilfeather of the National Parents' Council (Primary) believes children don't suddenly get responsible at 17, unless they've had experience of being responsible from an early age. "At age four, five or six, that could mean agreeing to let them watch a particular TV programme - to go and turn it on themselves and switch it off when it's over. By the time they're 12 or 13, you should be able to leave them alone in the house for an hour by themselves, with ground rules like `don't open the door'."

However, the issue of whether or not to let an older teenager stay alone in the house overnight, or for longer, has as much to do with their vulnerability as their behaviour, Kilfeather says.

"You should be training them all along to take responsibility for themselves; eventually, you have to face the reality that you can't know what they're doing all the time when they're teenagers, so you have to give them a moral framework and a sense of self-esteem.

"By Junior Cert year, you should be able to let them go to discos knowing they'll have a sense of responsibility about their own behaviour; you should be able to let them out knowing they won't come home drunk."

All that said, a 17-year-old is still very vulnerable if left in charge of the family home for overnight or longer, and the issue is how he or she might cope if anything goes wrong. "At the age of 17, I'd says it depends. Even if you have children who've gained and earned trust, I'd feel uneasy about leaving them alone, say, for a week." As Mitchell points out, parents will have to take into account other matters, like where they live, how isolated or otherwise their house is, whether you have neighbours your child could easily turn to for help.

Both he and Kilfeather agree that by the time children are 18, it's time to treat them as adults. And who knows? If you let them play house often enough, they might start to give you more help with the housework. As if.