The answer to this question is contained in the plan document. Write to your employer and ask for (i) a copy of the most recent plan document and (ii) a copy of the most recent summary plan description. If the employer fails to provide the documents w/in 30 days, then they become liable for a penalty of up to $110 per day for each day they fail to provide the documents.
Take these two documents to an attorney who specializes in ERISA matters, and s/he will be able to review the documents and answer your question. There is no general answer to this question -- it is entirely specific to your plan. FYI, it would be absolutely legal for your plan to make you wait until you reach retirement age (usually 65) before allowing you to receive a distribution.