When grown-up offspring are still part of the household

One of the phenomena of raising children today is that the job doesn't always end when the child reaches adult status at the age of 21 or 22. More and more, because of tight finances and an uncertain job market, grown-up offspring are deciding to live at home ''for a while,'' an arrangement that can sometimes stretch over a period of years.

In addition, the ''revolving-door syndrome'' is becoming popular: Parents help a grown child pack up amd move out, then convert his or her bedroom to a comfortable den. Six months later, the son or daughter is back - the job didn't work out, charge cards are overextended, mom's cooking has been missed - and the den furniture goes back into the attic.

Parents sometimes resent this intrusion, then feel guilty about their resentment and wonder what their proper role ought to be.

It's obvious that parents need to shift gears when sons and daughters grow up , moving from a parent-child relationship to an adult-adult tie in which each can enjoy the other without the burden of physical or emotional responsibility. This shift always takes time, but it's more difficult when grown-up offspring are still part of the household.

Occasional upsets aside, are there guidelines for living harmoniously with young adults?

Experts say a young adult should be free to make his or her own decisions - including choice of job, life style, curfew - provided those decisions do not infringe on the rights of other family members. No rules are really necessary among adults if consideration is uppermost in everyone's conduct.

The 23-year-old who notifies mom when he or she will not be home for dinner; takes care of laundry, appointments, and errands; and helps with routine chores can be a pleasure to live with, even if he or she does occasionally stay out late.

The son or daughter who constantly disrupts the family environment through quarreling, noise, or refusal to help when needed is demonstrating immaturity, and parents shouldn't tolerate it. ''The house belongs to you,'' one counselor constantly reminds his middle-aged audiences, ''and kids should be told to either shape up or ship out. This is painful, but to do otherwise is to retard your child's growth and keep everyone in a holding pattern.''

Experts also agree that adult offspring should pay board, although many parents are reluctant to bring up the subject. Even though parents may not need the money, even though the working son or daughter has a meager budget, grown children should not be allowed to sidestep financial responsibility. Parents who continue to support a working child keep him in a dependent position and shield him from the real world.

Ideally, board should be a set amount each week. It should not be dependent on a youngster's other bills and not viewed as a ''payback'' for mom and dad's care through the years, but should be seen as an indication of maturity. And the subject should be approached positively - ''It's great to have another adult in the house!''

If a grown child is paying for a college education or saving for a wedding, board need not be paid financially. Instead, it can be covered by increased home responsibilities. A student can cook dinner each night for a working parent or maintain the family cars or take over summer yardwork. These acts demonstrate adult responsibility, which is also the purpose of paying board, and they can be substituted when tight money is a legitimate excuse.

A young adult in temporary trouble should be welcomed home, but guidelines for the stay should be spelled out soon after bags are unpacked. As one parent put it, ''Everyone's entitled to make mistakes, so I've told our grown kids that we'll be here for them to fall back on. But they don't come back as guests. They come for a limited stay. And they don't return a third or fourth time.''

While these conditions may seem strict, they do clear the air, letting everyone know what to expect. They allow parents to help, without undertaking complete responsibility for the situation. They allow offspring a temporary refuge as they continue working toward complete independence.

''Letting go'' can be challenging - and even harder when young adults want to stay. But this transition time can be rewarding for everyone if consideration and candor are the cornerstones of family life.