Windows do not always wear at the same rate. Some windows may suffer damage and need to be replaced after 10 years, while others may last 30 years or more. The best way to know whether your windows need to be replaced is not to rely on age but to instead do an inspection of the windows to see what type of condition they are in. There are three steps to a basic window inspection.
1. Open, Close, and Lock the Window
Try out all of the windows latches and locks. Is there any resistance as you try to open and close the window? Does it stay open on its own after you open it, or does it try to close again? Do the locks truly engage and secure the window? If you have trouble opening, closing, or latching the windows, you might just need to replace the hardware. But often these issues are due to deterioration of the material that is holding the hardware in place, which indicates you need new windows -- not just new hinges or locks.
2. Look for Mold and Moisture
Inspect the inside and outside of the windows for any signs of mold, rot, or moisture. If there are little hints of moisture along the inside of the window sill after the windows have been closed, this is an indication of leaks. Also, look between the window panes (assuming you have dual-pane windows). If there is condensation in this area, that means that at least air is being allowed to pass through the windows. (Then the air cools and water condenses out of it.) Condensation between the panes means your windows are not efficient, and it can also lead to mold growth, given enough time. This is a window that should really be replaced.
3. Do the Incense Test
Finally, you should test each window to see whether air is leaking in around the glass. If it's a cold, windy day, you may be able to feel air leaks just by standing in front of the weather. But on a milder day, you will want to conduct an incense test. Light a stick of incense, and pass it over the crevices between the glass and window sash. If the stream of smoke waivers, that means air is coming in around the window, which is a good indication that the window needs to be replaced.
For replacement windows, contact your local window contractor.