The significance of her daughter Ciara's 18th birthday crept up on Yvonne by surprise. She hadn't thought it was an extra-special occasion until Ciara started dropping hints that she hoped for a big present for what she saw as a milestone in her life.

Preferably, a car.

It wasn't just watching American sitcoms, where cars feature regularly as 18th birthday presents, that had given her this notion. She actually knows a couple of students in her own school - an ordinary middle-class school in a large Irish town - who have received cars for their birthdays in recent months.

They are still the exception. "A lot are getting driving lessons, or money - £300 to £400 - for clothes and a holiday abroad. Another received a mobile phone," Ciara says of her peers. Meanwhile, Yvonne, who had been thinking along the lines of some nice jewellery and perhaps a modest sum of money towards a college fund, is amazed.

It was news to her that it has become quite usual for many Irish families to celebrate a teenager's 18th birthday in style.

As far as she was concerned, the 21st birthday was the big one. Says Yvonne: "I can still remember my 21st. We celebrated it at a hotel in town, and all my aunts and uncles and friends from work came; I've still got the gold jewellery my parents gave me as a present."

Yvonne is not alone: many parents are not aware that in the past few years, a teenager's 18th birthday has taken on special significance. No one is quite sure when or how it started, but it seems to be a fact of modern Irish life.

P J Hynes of the Magic Carpet pub in Stillorgan, Co Dublin, reports a growing number of requests from people who want to hold 18th birthday parties. "I'd say it's something that's developed in the past two or three years; the demand certainly wasn't there five to eight years ago."

Hynes is happier if these are family occasions, with parents in attendance - "it's less messy." The problem, of course, is that many 18year-olds have friends who are younger "and they get cross when we turn them away".

It seems that few parents give parties at home for their children's 18th birthdays, perhaps mindful of horror stories of teen parties that get out of hand. Parents also face the drink dilemma if they are inviting a mixed bunch of 18s and under-18s to their house for a party. Ciara plans to mark her 18th birthday by going out to dinner in a nice restaurant with her girlfriends; later they'll go to a club.

John Whyte of the National Parents Council is inclined to dismiss the notion of a lavish 18th as just another invention of greeting-card manufacturers. But psychologist Marie Murray, co-author of The Teenage Years, says there is something important going on here; it is important, she says, to celebrate either the 18th or 21st birthday of a child properly.

The 18th Birthday has taken on a special significance for teenagers nowadays, Murray agrees. Whether celebrated or not, this birthday does mark a real transition from childhood to adulthood in our culture. No matter that many children will be financially largely dependent for another five years or more; "it's a psychological reality. Children expect to be less accountable, more independent after 18. Celebrating an 18th birthday is a kind of acknowledgment that the passage to adulthood has been shortened by three years."

Murray agrees however that There is, however, confusion about all this in our society, with some families celebrating a child's 18th birthday, some their 21st - and some both. This reflects a genuine confusion about when adulthood begins: after all, most Irish teens celebrate their 18th birthday during their Leaving Cert year, before they have taken their first steps into the world of work or third-level education.

"The age at which we define young people as adults can vary so widely," Murray says. "In the US, for example, you can drive at 16 but drink at 21 in some states, and in others, handle guns when you're under 16." Here, she agrees, it is sad but not surprising - given our alcohol-tolerant society - that teenagers see being able to drink legally as one of the most significant markers of the transition to adulthood.

As for the 18th v 21st dilemma, Murrays says parents should anticipate it some time before the 18th birthday and agree with their child which will be the major celebration. "It's better than having two inadequate parties."

For Murray, the issue isn't how much you spend or how expensive a present you give your child (teenagers might not agree). What matters is that you do celebrate this important rite of passage, either at age 18 or 21, in a way suitable to your own wallet.

"It carries a huge message and, like Confirmation or first Communion, is a special kind of celebration that includes the extended family. It's a statement made by parents to mark a child's adulthood, to say that you now perceive them as being adult."

The interesting thing is that in their hearts, many parents believe that this is true at 21 - whereas the impetus to mark this rite of passage at 18 seems to have come from teenagers, making a statement to adults about their eagerness to grow up.

Meanwhile, deep in their hearts, many teenagers still harbour the - dare I say childish - fantasy that they'll wake up on the day they turn 18 to find a car, or some equally unlikely present, figuratively gift-wrapped at the foot of the bed.

Dream on.