The seal on most double-glazed units will age and eventually fail and condensation between the panes will form, some factors can influence how quickly this happens. 

fortunately in most cases failed or misted glazing units can be replaced quickly and easily with no mess or fuss at a fraction of the cost of replacing the whole window or door.
 

The most common causes of premature failure are: 

- A poor quality or faulty unit 
- Poor, blocked or no drainage causing the unit to sit in water
- Prolonged direct Sunlight, South facing for instance.