C.P.
On the age limitation, I think this has to do with your baby's development and not the stroller itself. They don't recommend jogging with small infants (under 6 months is what I have heard) because their necks aren't strong enough for the jostling and bumps. As a result, not many jogging strollers even accommodate infants under 6 months. There should be nothing wrong with a brisk walk, though!
This site has some great search and comparison features that might help you find the right stroller: http://www.joggingstroller.com/Shop-by-Activity/Jogging/i....