The BLS document Self Employment In the United States : an update from 2004 has some data on self employment at that point. They show some previous trends over the previous years. I also found the current data from 2009 which shows the total # of self employed and total # of employed in Table 15 from which I figured the %'s by age group for that year.
Fairly flat for a few decades
Table 1 of the BLS document has data going back to 1948 showing the overall self employment rates. I'm looking at just the non-agricultural industries. Here is a graph of the historical trend:
From the '40's to the 60's the rate of self employment dropped off quite a bit. It was 12% in 1948 and had hit 6.9% by 1970. Since the 1970's the rate of self employment has been relatively flat around 7% within +/- 0.5%. As of 2009 the rate was 6.5%.
Older are more likely Self Employed
Self employed are usually older and younger workers are much less likely to be self employed. That has been very consistent over the years. Table 5 of the BLS document gives the unincorporated self employment for nonagricultural industries by age, sex and races from 1989 to 2003. Table 6 in the document also lists the incorporated self employment rates. I'm focusing on just the unincorporated self employment which is the larger group. The graph below shows the %'s of self employed per each age group for 1989-2003 plus 2009.
Without variation people are more often self employed as they age. One notable trend in that graph is among people 65 years or older from 1997 to 2003. In 1997 the self employment rate for people 65 or older was 19.5% but it had declined to 15.3% by 2003. Thats a significant drop in the rate of self employment for people in that age group over just a few years. Other age groups also showed declines in self employment rate in that time period but they were not as severe.