Despite the frenetic screams of the faithful at Wolf Trap last night, Shaun Cassidy has begun to live the numbers nightmare that befalls all teen-age products of our prime time.

Like his brother David before him, Cassidy has reached an awkward age. Once boy singers used to worry about their voices changing; now they have to worry more about their audiences changing. The Hardy Boys television package had a subtle warning -- sell before age 21. Fast approaching that mark, Shaun Cassidy is trying to evolve into a serious actor. Musically, he's become a more than passable, though uninventive, rocker.

Audience residuals will provide him with a following for a while, but there is no stepping back over the time line. Cassidy's youth is non-returnable: in a few years, that of the majority of his current audience will be equally spent. What continues, will depend on Cassidy's serious commitment to music and to acting. There is evidence that the young man is more than willing. It would be interesting to hear him sharing the stage, unannounced, with another act. Forgetting images, audiences would discover some blazingly direct rock 'n'roll.

For tonight's blues and bluegrass show at Wolf Trap, Muddy Waters has canceled due to illness. He will be replaced by Seldom Scene, a local bluegrass group that has won national following. Bill Monroe remains on the program.