In reply to rajicheena:
I think you are overthinking this. Heart rate is a very individual thing. As long as you don't feel unwell I don't think there is anything wrong with maintaining a heart rate of 155-160. The key with a heart rate monitor is to learn what is normal and what is abnormal for you and compare what you are seeing with how you feel. Don't try to compare yourself with everyone else.
As others have said, wrist based monitors are less accurate than chest straps - however I have found mine to be consistent if not accurate, so I can still use it to track myself as long as I don't expect my measurements to be absolute.
I am 36. My baseline measures are similar to yours. Resting just under 60bpm, walking briskly around 120-130bpm, might hit 140 marching up a munro with a big pack. When I run, an easy jog will be around 160 bpm, I'll maintain around 175 bpm over the course of a half marathon race and around 180 bpm in a 10km race. So what you say actually sounds perfectly normal to me!