It's simple: Give me a reason to write back!
And for that it needs more than just 5 standard sentences. I have to admit I delete the most of all first messages because the people don't really put something in them. And I'm not here for gathering friends either. If we don't exchange messages regularly I don't see a point in adding you to my friend list.
The age should be more or less around my own, but it doesn't really matter as long we have something to talk about.