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 told AFP.

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 told AFP.

One man who has got this down to a fine art is 54-year-old Austrian businessman Hermann Paschinger, creator of the "50plus" 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," Paschinger told AFP.

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.

"We don't mention the width of doorways for wheelchairs," he said.

People over 50 "hate being told they are getting old."

Other tourism firms have jumped on board, including Denmark's 65-Ferie, German giant TUI's "Club Elan" and Thomas Cook's "Club Vital."

And the silver-haired tourists have little fear when it comes to choosing faraway locations for their holidays, said Thomas Graune, founder of German firm Studiosus, offering cultural trips for the 50 to 60 crowd.

"These people have increasingly exotic tastes. They've already seen Italy and Greece. Now they want to go to Ethiopia, Yemen or China," he said.

Volker Schmidt, from the German lobbying group Seniorenring complained that "some holiday firms only think of seniors when it comes to the low season."

However, with the baby-boomer generation approaching pensionable age and medical advances meaning people are staying healthy for longer, the market is growing quickly and firms are rapidly wising up to the potential.

"Happily, things are changing," said Schmidt.