Welcome, Prashant!
Before I give you a generic answer, can you please clarify which product in the SAS CI family you're using, and which version? There are certainly different ways to answer this depending on those two factors.
In general, though, Marketing Automation (MA) allows this on data items with their "level" attribute set to "Interval" in the information map by presenting you with the option to set a minimum and a maximum. In recent versions (6.X), you can just type "> 18" in the right hand side of the selection criteria, but in older versions you'd have to click on the "..." button in that textbox and put a minimum of 17 and a maximum of 26 (or use * to mean no limit).
If you're asking about a data process node in Real-Time Decision Manager (RTDM), then that's only possible in 6.5, if memory serves.