"Irreconcilable differences" were cited by Robyn Gibson, wife of actor Mel, who filed for divorce last week. If celebrity divorces, with their accompanying and usual public airing of dirty laundry are the stuff of tabloid dreams, Gibson vs Gibson is particularly intriguing. Firstly, there's the money issue. The actor's fortune is estimated Gibson's fortune estimated at around $US900 and under Californian law, property and earnings are divided 50/50 where there is no pre-nuptial agreement, which means Robyn could be headed for one of the biggest pay outs in Hollywood history.

But the other interesting question is, why now? Why throw in the towel and divorce after 28 years of marriage? The couple, who wed in 1980, have seven children and are seeking joint custody of the youngest son, Tom (9). Their marriage was perceived to be a strong one, certainly by Hollywood standards, and had survived his alcoholism and rumours of infidelity on his part, weathering the storms when he was accused of being homophobic and anti-Semitic. Gibson referred to his wife as his 'rock' and 'a saint' but it would appear that her fortitude had been pushed to the limit when paparazzi pictures emerged last month of Gibson embracing an unidentified brunette at his holiday home in Costa Rica. She's rumoured to be a Russian called Oksana and since then three other women have emerged as being the possible catalyst for divorce.

Far from being an unusual example, the Gibsons join a long list of high profile couples who decide to separate after decades together. Formula One boss Bernie Ecclestone and his wife Slavica divorced last month after almost 25 years of marriage, on the grounds of his 'unreasonable behaviour'. Famously mismatched in terms of height (he is 5ft 4in, she 6ft 2in), the couple seemed to have enjoyed a harmonious union but in court papers Slavica said her husband's behaviour "has caused me stress and anxiety."

Oscar-winning actor Morgan Freeman (71) announced his divorce only days after being injured in a car accident last year. He'd been married to Myrna Colley-Lee, for 24 years.

Here at home, broadcaster Gerry Ryan and his wife Morah regretfully announced their split after 26 years of marriage and other surprise separations included Robin and Marsha Williams (18 years); golfer Greg Norman and Laura (25 years) and Harrison Ford and Melissa Mathieson (18 years).

As to why these couples ended up in divorce court instead of resolving – or ignoring – their differences and entering their twilight years as a unit, it's a fact that with our longer life expectancy comes greater expectations of our older years. It's now a vibrant age and not just the beginning of the end. There's that great cliché, the midlife crisis, although as Wall Street Journal columnist Sue Shellenbarger writes, it's only a cliché until you have one.

According to family therapist David Kavanagh, this is a time of re-evaluating your life. "It makes you pose questions such as: what am I doing, am I happy with my career, am I happy with the decisions I have made? Should I have done something different? And can I do something different now to add more meaning to my life?" Kavanagh says. "People can find themselves challenged by these questions and imagine or believe that the answers are outside of the marriage. People tend to move apart from their partner at this point unless they make a conscious decision to reconnect. That takes courage and bravery and it's hard for a lot of people."

Kavanagh says that in many cases women are the instigators. "Women get married at a young age, have children, have families, do all that they think they're supposed to do and then realise they're 45 or 50 when they're kids have grown up that 'Actually, I'm completely unhappy. I've sacrificed my career, my goals, my dreams, for my children and my husband and now it's time for 'me' time'."

The Gibson divorce will be amicable, according to friends of the couple, but is it necessarily the end? Sometimes there are happy endings. Singer Paul Young and his wife Stacey were reconciled after a three year separation. The former model had left her husband after 20 years of marriage, but they were happily reunited last month.