Final answer:
In the 1980s, the Reagan administration's war on drugs indicated a response to rising drug use, specifically crack cocaine. However, without specific survey data, it's impossible to conclusively state the trend in the percentage of young adults using illicit drugs during this decade.
Step-by-step explanation:
During the 1980s, the use of illicit drugs among young adults (age 18 to 25) showed significant trends that are critical to understanding that era. The war on drugs began under the Reagan administration, casting a spotlight on substances like crack cocaine. Amidst campaigns like "Just Say No" and increasing concerns about drug-related crime and violence, there were societal changes and legislative measures that resulted in stiffer penalties for drug offenses. Although the question about drug trends can be complex, denying judges' discretion in drug case sentencing and the introduction of policies such as the three strikes laws indicated a governmental response to increasing drug use among certain populations.
However, data from surveys or studies that specifically measure the percentage change of drug use among young adults during the 1980s are not provided in the reference information. Thus, we can discuss the context and policies of the era, but cannot definitively say whether usage increased, decreased, remained stable, fluctuated wildly or remains unknown based solely on the given information.