Which rule was useless in Jonas's community?

The rule against children under nine riding bicycles was useless because everyone broke it.
At the age of nine, everyone gets a bicycle in Jonas’s community.  It is one of the rites of passages and the ceremony gift given at the Ceremony of Nine.  The bicycle is considered an important symbol of growing up and independence, limited though it is.

The bicycle, at Nine, would be the powerful emblem of moving gradually out into the community, away from the protective family unit. (Ch. 6) 

Thus, bicycles are very important to the community.  Everyone has one.  They use them instead of cars, even as adults.  The Nines look forward to getting their bicycles, and so do their families.  This is why older siblings secretly teach them to ride the bikes while they are still Eights, so they can use the bicycles as soon as they get them at the Ceremony of Nine. 

It was one of the few rules that was not taken very seriously and was almost always broken. The children all received their bicycles at Nine; they were not allowed to ride bicycles before then. But almost always, the older brothers and sisters had secretly taught the younger ones. (Ch. 6) 

The reason that the rule is so easily broken in a society where there are such strict consequences for rule-breaking is that changing the rules is so difficult in their society that they have to just go the unofficial route and ignore the rule.  Supposedly committees have to “study the idea,” and change is so slow that a rule change happens rarely, if ever.  So the community makes its own change.  Everyone looks the other way, and the simple rule-breaking is considered harmless because it is community-wide.  It becomes a joke instead of a serious issue.