What is career length average of Pro football player?

"There is a little bit of a misrepresentation or a misunderstanding on that. Frequently, it is said that the average career is about 3.5 years. In fact, if a player makes an opening day roster, his career is very close to six years…If you are a first-round draft choice, the average career is close to nine years. That 3.5-year average is really a misrepresentation. What it adds is a lot of players who don't make an NFL roster and it brings down the average."