I am looking for economic statistics that break down how much money
Americans spend by age group starting with 18 year olds.

Hello ruling-ga

Thank-you for your question.

The official figures for the expenditure of US citizens is available
from the Department of Labor's Bureau of Labor Statistics.  The
homepage for this information is here:
http://www.bls.gov/cex/home.htm

The latest information available is for 2003 and is broken down thus:

All ages       $40,817
Under 24       $22,396
25-34          $40,525
35-44          $47,175
45-54          $50,101
55-64          $44,191
65 and over    $29,376
65-74          $33,629
75 and over    $25,016

You will not that the lowest age range is under-25's.  Perhaps not
surprisingly the largest spenders are those in the 45-54 age group.

Full information including further breakdown of this information into
what this expenditure was on can be found here:
ftp://ftp.bls.gov/pub/special.requests/ce/standard/2003/age.txt

If you require any further information on this subject please do not
hesitate to ask for clarification and I will do my best to respond
swiftly.