The reality is that fantasy sports are an inexact science. It is entirely based on data yet no one can accurately predict anything. That's why these concrete rules seem to fluctuate. A couple of years ago, for example, it was crazy to draft a QB first overall then it wasn't now it is again.
But there are some philosophies that just won't go away and I want to delve deeper to see if they should or if they are actually good rules to stick to.
1. Don't take a 30-year-old running back:
After this season, this age-old rule will either be even more powerful or completely shot. There are two guys who are 30 now and are expected to be solid number 2 running backs: Steven Jackson and Frank Gore. If they perform as expected, this rule will likely be thrown out the window. But if they disappoint, the critics will say, "Yep, that 30th birthday sure was the turning point."
This shows just how arbitrary the rule is. The fact that running backs often fall off after they turn 30 is a classic correlation and has nothing to do with causation. There's no magical event that signals the decline of a running back. One back's 28 is another back's 32, it varies so much by individual player that this rule is useless.
Let's start by recognizing the fact that there are very few running backs 30 years or older to begin with. Last season there were five backs that were at least 30 who got more than 25 carries. That's not a very big sample size and two of them, Cedric Benson and Ronnie Brown, did not ultimately see much of the field (not because they're old, but because they haven't been consistently good at playing running back)
Of the other three, Michael Turner (age 30), Willis McGahee (31), and Fred Jackson (31), only Turner played in every game. McGahee and Jackson both missed time but were fairly productive when they did play. And both have long injury histories, so it is dicey to claim that their age made them injury prone
In both 2010 and 2011, there were nine "old" running backs that got carries. In 2010, Ladanian Tomlinson (31) and Thomas Jones (32) finished 19th and 20th respectively in the NFL in rushing yards, ahead of in-his-prime Frank Gore.
In 2011, McGahee (30) finished ninth in rushing yards.