If you look up the G-INFO database
http://www.caa.co.uk/application.asp...ype=65&appid=1
you can find information about date of registration, ownership, hours flown and date built of any aircraft registered in the UK.
A lot of the BA 737's for example are registered G-DOCD, DOCA, DOCO, DOCU and so forth. They are on average 15-16 years old.
Age is not a problem for airplanes normally, as long as they're maintained properly.