The only thing re the longer cutoff date is that older bike will 99% of the time mean more $ on maintenance and that sort of defeats the purpose of a cheaper/easier intro into racing. 8 years might be the magic number but good luck finding an 8yo bike you can hang on to for 2-3 years without spending a fair bit refreshing things at some stage.

Having said that, I'm sure someone will point out that racing is not cheap and bla bla bla... not denying any of that just pointing out that if you buy an 8yo bike, there's a good chance it will cost more than buying a current model in the long run and then when you try and sell it 2-3 years later it probably won't be a nice financial experience.