Normally, I'd say yes (I'm on a 2 year old iPhone 4s)
However, now I don't really know...
I just had my phone replaced by the genius bar. It's as good as new.
And I look around, and flagship devices still for sale today are still running the A5 chip - iPad mini, iPod Touch... iOS 7 is running completely well on the A5 chip. It was probably designed with the A5 in mind.
This is in contrast to previous updates whereby the 2 year old devices would be left out in the update - the then 2 year old iPhone 3G did not get iOS4's multitasking and it was laggy as heck... the two year old iPhone 4 did not handle iOS 6 very well - it didn't get panorama, it didn't get turn by turn, no Siri and not cellular facetime...
But iOS 7 makes the 4S feel like the current flagship phone. It doesnt feel like a 2 year old phone...Why would you even get an iPhone 5??? It has no added features except Airdrop.. And I'm not reading any rumors to suggest anything groundbreaking with the 5S....
Unless the 5S blows everything out of the water, I may just stay with the 4S for another year and go for the iPhone 6...or 'gasp' the next nexus (if the iPhone 6 still seems stale)???