You will defiantly be on the younger end of the curve at Desire Pearl - actual age grouping is really dependent upon the week that you will be there
As to ages of those who go to Desire the forum POLL on ages is really pretty accurate for both resorts - this to, of course, will vary depending on any given week
You can check out this thread where your question has "sort of" previously been posted by another forum member.
Average age of guests & international diversity?