Who gets jet-lag?

Almost everyone on a long flight suffers jet-lag to some degree.
Studies by Upjohn and Condé Nast have shown 93-94% of long-haul travellers experience it, and a New Zealand study showed a similar proportion of flight attendants on international flights suffer from its effects despite being accustomed to long-haul travel.

It affects passengers even more than the flight professionals, firstly because they are generally less accustomed to the factors causing jet-lag, and secondly because they are confined to a cramped space for long periods. There are also other factors such as the lack of fresh air in passenger areas.

Not all people to the same degree
Young children often seem immune. People who normally stick to a rigid daily routine or are bothered by changes of routine, are often the worst sufferers. People whose normal lives involve highly varied routines can often adjust their circadian rhythms better and adapt to a disruption of normal eating and sleeping patterns. People who get to sleep easily may also cope better with the adjustment.

People crossing multiple time zones
The length of the flight is not the critical issue. The most important single factor is how many time zones you cross. People can suffer jet-lag just crossing the United States from New York to Los Angeles (three hours' time change) but would be much less affected by flying from New York to Bogota in Colombia, a north-south flight of even longer duration. But there are also other factors besides crossing time zones that contribute to jet-lag, and these may be experienced even on north-south flights.

People in poor pre-flight condition
If you are not fit, rested and healthy you will probably suffer more jet-lag than others on the same flight. Stress and loss of sleep before your trip will will make it worse. Pre-flight partying and over-consumption of alcohol will do the same.