Allegiant's fleet average age is 22 years, Delta's is 17, American's and United are both 13.6 years. So, a 24 year old airplane being in service is, yes, older than most but not unheard of. But of equal importance IMO, if your an airline that is retiring your Boeing aircraft and replacing them with European Airtrash models, having a younger fleet average is hardly something to be proud of.