Bruce Deacon recently won a 5k race in B.C. in 15:52, I would say that is respectable. He's 46 years old.
For the average 40+ runner I would say 18-19 seconds is something to be reasonably happy with.
18-19 seconds for 5K? That's moving. That's about 16-17 kilometres a minute, or 1000 km/h; a shitload faster than Bruce, Mo or Kenny B!