Well, no, not exactly.
The law permits employees who work under 1000 hours a year to be excluded. Employees who are under the age of 21 not only can but I believe must be excluded.
Beyond that, a 401k plan has to pass a discrimination test which more or less requires that all other employees must be allowed to participate. That being said, it is possible for a plan to be written in such a way that it is only applicable in certain situations - for example, as part of a senior executive benefit package.
If this does not answer your question, please post back with details.