"The F-15C Eagle remains the USAF's
front-line fighter-interceptor today, and the Air Force will depend
on them for years to come, but most of them are already fifteen to
twenty years old. The F-15's planned replacement, the F-22, is
making slow progress; its high cost is one of the most serious
limitations on early adoption of the new
airplane."
Source: ANN's Kevin O'Brien, in his story about
the issues revolving around the USAF's F-15 fleet, after a recent
crash.