Someone asking about 13-year-old Jewish children got this as a partial answer:
The problem with getting demographics is that there is no agreement on them. The Census, for instance, is prohibited from asking religion questions. I have reports indicating they’re declining precipitously and others with a small increase.
ARIS indicates that the adult Jewish population has declined from 3,137,000 (1.8%) in 1990 to 2,680,000 (1.2%) in 2008. They note that definition is practicing Jews, not “ethnically” Jewish. In 2008, 49% of Jews were male, 51% female. 21% are 18-29, 28% are 30-49, 33% are 50-69 and 18% are over 70.
Conversely, Pew Research shows 1.7% of adult Americans are Jewish; Reform 0.7%, Conservative 0.5%, Orthodox- <0.3%; rest are "other" or not specified. The AJIS study from 2001 suggests that there were roughly 2.5 million Jewish children.
From ARDA: Judaism by county (2000) and by metro area (2000). For 2005, it estimates a total of 5.7 million Jews in the US, 1.7% of the total population.
By comparision, here's the world Jewish population by country.
Having found bar mitzvah website, I was frankly happy to find this relevant quote here: "I doubt very much there are any statistics on the number of Bar/Bat Mitzvahs are in the US. Each synagogue has their own set-up and there is no central database I know of even for the main denominations… What you'd need is a census of Jews turning 13 in any given year - and that is not really available anywhere you could rely on. Some portion of THAT number would have a bar/bat mitzvah ceremony."