Wood windows that were built 100 or more years ago (the double-framed variety) were built to last; however, those that were built 30 or 40 years ago were not. These windows are subject to wood rot and window shrinkage. The energy efficiency of the glass is often under question in windows of this age. For this reason, once these more recently constructed wood windows start showing these signs of aging (wood rot, cracking, mold, etc.) it is time to think about replacing them.