WASHINGTON. Aug. 31--- American youngsters may have to be prevented from crossing alone into Mexico where drugs are readily available to them the Senate Crime Investigating Committee said today.

The committee said in its final report that many youngsters were visiting towns, across the Mexican border "for the sole purpose" of obtaining drugs such as marijuana and heroin "which can be purchased in the street."

The committee urged the Government to study this problem. It added:

"The answer may be in prohibiting minors to cross the border when not accompanied by their parents."

The committee also said additional hospitals might be needed for treatment of drug addiction. It asserted that United States Public Health Service hospitals at Lexington, Ky., and Fort Worth, Tex., might prove inadequate when the public became fully aware of their availability to volunteer patients.