Press barons, sailing, 21st-century life, and more; Bright view of century ahead; The Good Years: Your Life in the Twenty-First Century, by Caroline Bird. New York: E. P. Dutton. 288 pp. $15.95.

The most certain fact about your life in the 21st century is that you will enter it 18 years older than you are today. For most of us, this means being middle-aged or older. But according to Caroline Bird, this period doesn't have to mean decline or decay - it can instead be vibrant and expansive.

Bird, herself a woman of mature years, states flatly that ''the stereotypes of age are myths.'' In place of ageism, she substitutes the concept of ''agelessness'' where attitudes, not calendars, are of central importance. She compares the opportunities of retirement to those of college, noting that ''both are periods of transition during which individuals are free to experiment with new priorities.'' She describes the varied, fulfilling careers of many of today's older Americans, citing them as pioneers of the next century's senior life styles.

Bird's liberating views about aging are the strongest feature of her speculations about the decades ahead. Rather than take a technology-oriented Toffleresque approach, Bird mostly projects current social trends - from an unflaggingly optimistic perspective. Some readers may be dismayed by her casual attitude toward sex or by possibly tongue-in-cheek suggestions, like the one that we repudiate the US national debt. But if Bird's vision of the future is at all correct, take heart; your best years are yet to come.