This is from bowl.com when you look up averages.

"Adult averages with less than 21 games or youth averages with less than 12 games are not officially recognized by USBC. "

This is from the Rule book:
118c. League Entering Average
Entering averages are determined by league rules. A bowler who does not have an entering average,
or until they meet the requirement(s) defined by league rule(s), average is determined by dividing the
total number of pins by the total number of games bowled the first league session. (See Rule 100g
for handicap.) When a bowler fails to complete his/her first game in order to establish an average, the
frames bowled will be declared null and void and a vacancy score used.
NOTE: The following are options for entering averages:
 1. Highest Average: The highest average in any USBC league in which a player competes in
 one season.
 2. Composite Average: The average of a bowler who bowls in two or more USBC leagues in a
 season. The average is determined by adding the total pins for all of the leagues and dividing
 the result by the total number of games bowled in those leagues.

Most leagues that i've seen use last years league average which was usually the  minimum of 21 games average,  But according to the rules the league could set another figure.

For USBC  Tournaments USBC Rules:
319a. Conditions that Apply
The following conditions apply to averages in handicap or classified tournaments, unless the
tournament rules state otherwise, except that only USBC league averages shall be accepted. (See Rule
319c for average adjustment.)
1. The bowler is required to submit the previous season’s highest average of:
 a) A single USBC league average based on a minimum of 21 games; or
 b) A composite average of all USBC leagues, including leagues with less than 21 games bowled.

It's 21 games in a league, even if your a sub.
I booked an average as a sub several times now.
If you don't want to book that average, bowl 20 games and stop.
[edit]
The league secretary is responsible for uploading book averages and following the rules at the end of the season. But I've seen many cases where I bowled or subbed 1, 3, 6, 9 games in a league and it was uploaded as an average to be reference as part of a composite average.

At my center, we have the book, updated every year. We go to the control desk and we do a lookup on the official USBC book, that has a computer print out of our regions members. There you'll find your name with a book average for league, and if you bowled in a sport shot league, your other sport book. The book is reprinted every tax day.

That's interesting!

I've never been to nationals but the possibility of being re rated is plausible and of course I believe your claim. So you get re rated, your balls are weighed and always incorrect requiring a weight hole, and your bag checked for USBC illegal stuff like cleaners.

I've heard of many leagues and tournaments re rating a bowler, and saying no way, your book can't be that low.

I'm not sure 21 games is the rule. Our league had a question about averages this year and our rule said highest average from any league. When we asked the USBC they said unless we said 21 games it could be any amount. So I'm not sure 21 is set in stone.

For league entering average, no, the league can do whatever they want, usually preferring to start with any average established in that league first for any duration, but there ARE rules for tournaments, that's more what I was referring to. 

Quote from: TDC57 on March 06, 2018, 08:11:08 PM

I'm not sure 21 games is the rule. Our league had a question about averages this year and our rule said highest average from any league. When we asked the USBC they said unless we said 21 games it could be any amount. So I'm not sure 21 is set in stone.

I would suspect that tournaments will make there own rules on the fly.

Your original question was for USBC sanctioned leagues I believe, so in hindsight I think the answer is:

"They can vote in a new rule and specification or choose to keep the existing rule and specification with any sub clauses or change to said sub clause agreed upon after the issue is cast for vote; and then after a majority vote of I, yes or raised hand gesture by those who physically attended the meeting".

I meant to word that in a warm fuzzy way but it came out more legal like.