In The Giver, why might minor rule breaking, such as riding a bicycle before the age of nine, be tolerated in the otherwise strict community?

The rule against bicycles is broken because it is a minor rule and breaking it is tradition.

Rules that are broken for everyone are unspoken rules of their own.  This is part of community conformity, which is fundamental to Sameness.  Traditions generated by minor rule-breaking turn into rules when they are codified by the committee.

The rule against children under nine riding bicycles is the one Jonas says is always broken.  Older siblings teach their...

The rule against bicycles is broken because it is a minor rule and breaking it is tradition.

Rules that are broken for everyone are unspoken rules of their own.  This is part of community conformity, which is fundamental to Sameness.  Traditions generated by minor rule-breaking turn into rules when they are codified by the committee.

The rule against children under nine riding bicycles is the one Jonas says is always broken.  Older siblings teach their younger siblings how to ride so that when they get the bike, they can ride it.

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. 2) 

You can see why the community would not mind this rule being broken.  One, it adds to efficiency.  Once the nine year olds get their bicycles, they can use them immediately.  Two, it is tradition.  Older brothers and sisters all teach their siblings to ride.  Breaking the rule is an unwritten rule.  Finally, allowing the rule to stand even though it is continually broken reinforces the idea that it is very hard to change a rule, so that no one will attempt to change rules willy-nilly.

Rules were very hard to change. Sometimes, if it was a very important rule--unlike the one governing the age for bicycles—it would have to go, eventually, to The Receiver for a decision. The Receiver was the most important Elder. (Ch. 2)

Jonas comments that the community will never change the rules against bicycles, because it is not considered an important enough rule to bother the Receiver with.  Instead, they will “simply fret and argue about it themselves for years, until the citizens forgot that it had ever gone to them for study.”  While they are fretting, the citizens develop their own unwritten rule about bicycles and the rule change is not really needed.