IRS Pub 554 states (click thru for the tableto read full IRS doc):
"Do not file a federal income tax return if you do not meet the filing requirements and are not due a refund. ... If you are a U.S. citizen or resident alien, you must file a return if your gross income for the year was at least the amount shown on the appropriate line in Table 1-1 below. "
You may not have wage income, but you will probably have interest, dividend, capital gains, or proceeds from sale of a house (and there is a special note that you must file in this case, even if you enjoy the exclusion for primary residence)