The key to your question is AVERAGE. So if someone tells you the 'average' length is 6', they are nuts....LOL!
I would say the average length for a 2 year old snake is 4-4.5 feet in length.
I have a pair of het albinos that are coming up on two years old in May and they are right at 5'. This is only because I have been pounding them more than I normally would over the last 6 months.
I JUST moved out a 5 year old male Hypo that was right at 6'.
I know people say they breed their boas at 18 months/6', but that is far from normal OR average.
You also have to look at this when you are asking yourself the questions you are. Just because an animal is an '02 and we are in '04, does not mean the animal is 2 years old. It could have been born in Sept or Oct of '02 which means it would just be over a year old. Big difference from 2 years.
I am sure these are all things you know of, but you asked. 
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Brian Oakley
Phoenix, Arizona
BrianOakley@cox.net