Whilst I also agree that 12 years is far too small a lifespan for buses there are a couple of factors that have to be kept in mind , firstly while it was the CIE companies that first set the 12 year age limit , the NTA has not only endorsed this , but has asked both companies to IMPROVE their fleet age profile meaning that the 12 year age limit may soon be lowered again to 10 years.
Secondly is that there are currently 180 2000 reg AV's , it is highly unlikely that there will be funding for any more than 60-80 buses per year for the next few years meaning that the 00 AV's will have to be withdrawn over 3 or so years , hence why the first AV's ( the mechanical troublemakers have probarbly been identified already) will be withdrawn along with the last RV's , also , a battered AV will get more on the secondhand bus market than a pristine RV would.