From Japan to Germany, senior citizens make up a lucrative and fast-growing opportunity in the tourism industry, with deep pockets, disposable income and free time. But just don't mention their age.

"The over-50s constitute a big market," Sybille Zeuch from the German Tourism Federation (DRV) said at last week's ITB in Berlin, the world's biggest trade fair for the multi-billion-dollar global holiday industry.

"And the 'Best Ager', 'Silver Ager', 'Seniors' or '50s and over', call them what you like, are the perfect guests. They have strong purchasing power, are interested in culture and are used to travel," she said.

But firms wanting a piece of this "grey market" face a major obstacle: how to attract seniors without them realising they are being aimed at because of their age.

"They are the hardest market to target," said Jean-Claude Baumgarten, head of the World Travel and Tourism Council (WTTC).

"The secret is to find a dynamic way to communicate, to make these people feel like they belong to a sort of club," he said.

One man who has got this down to a fine art is 54-year-old Austrian businessman Hermann Paschinger, creator of the "50 plus" badge certifying dozens of hotels in Austria and Germany as suitable for the older visitor.

"These are hotels with three to five stars, with a guarantee of comfort, relaxation, impeccable service, with an eye for detail and offering nature and cultural trips," Mr Paschinger said.

The brochures show couples bursting with health, vitality - and relative youth - happily engaged in activities like hiking or cycling, or relaxing next to a lake quaffing a well-deserved bottle of wine.