Below is a summary of Airport Express / Airport Extreme / Time Capsule compatibility and PeakHour:

Airport Extreme / Time Capsule 
Airport Extreme / Time Capsule 1st-5th Generation (2007-2012)Yes
Airport Extreme / Time Capsule 6th Generation (2012+)No
Airport Express 
Airport Express 1st Generation (2004-2008)No
Airport Express 2nd Generation (2008-2012)Yes
Airport Express 3rd Generation (2012+)Yes

If you're unsure on which model you have, download the free trial and run the Scan for SNMP Devices in the Getting Started.

A note about the Apple AirPort Extreme / Time Capsule 2013+

As of the this writing, Apple have removed all support for SNMP in their latest hardware, making them impossible to monitor (with any software). There is further discussion about the issue on Apple's support forum: Apple Support Discussion - SNMP and 2013 Airport/Time Capsule.

If you have one of these routers, you can still use PeakHour to monitor Macs, PCs, servers and other devices on your network using SNMP. See this article for step-by-step instructions.