Most athletic shoes have a useful life of between 300 and 500 miles. There are a lot of factors that can influence this. Physical characteristics of the user, terrain or surface used on and the activity itself as well as shoe design.
Unfortunately, shoes often look better than they actually are from the standpoint of their ability to support and protect your feet and skeletal system. Once the maximum mileage is reached, fatigue and injuries increase. Forefoot pain from loss of cushioning, arch pain from loss of support and shin splints and knee pain from lack of control of foot position.
Runners are typically more aware of the mileage they are putting in on a regular basis, therefore, tracking the shoe mileage should be easier. Those who choose to mix their activities may have a more difficult time; for both, this may be a time where the new fitness watches and applications can be quite valuable. Being able to track true mileage and then committing to replacing shoes at a specific interval can go a long way towards reducting complications from excessive shoe wear.
If you are experiencing a new onset of pain that seems vague or even acute, replacing your shoes, particularly if you are unaware of their mileage, may the first and best place to start.