In the online world, younger is usually better. A Web site that appeals to a 25-year-old users is seen as cool and cutting edge.

But for fund raisers, that younger-is-better thinking is actually counterintuitive, says Tom Belford, a marketing expert.

Mr. Belford, a veteran fund raiser and an author of the popular fund-raising blog The Agitator, says nonprofit groups are making a major mistake if they attempt to create lively sites that appeal to younger visitors.

“For most nonprofits, I suspect the target demographic is 55-years-old or so, with no upper age limit and maybe down to 45,” he says. “If you’re one of those nonprofits, maybe you (or someone who is 55) should take another look at your Web site.”