You thought it was you who was getting old. well, "Sesame Street" is being put into a museum.

And it's only 10.

On the other hand, the Smithsonian isn't your average dusty-shelved cultural graveyard. Yesterday, the halls of History and Technology quivered with the shouts of 30 nursery schoolers from Holy Trinity as they clustered before the actual "Sesame Street" set, complete with Big Bird in its nest, Oscar the Grough in his gatbage can, the familiar brownstone front and evan a piece of New York sidewalk.

"Hi!" said Bob McGrath, who plays Bob.

"Hi!" the nursery schoolers shouted.

"What music is that?" he asked, turning on a tape.

"SESAME STREET!" they screamed.

He sang some songs with them. Alaina Reed, who plays Olivia, sat with children, calpping and stamping when they did. TV cameramen leaned into the scene. A few feet back, a very small boy in a Big Bird T-shirt and tiny saddle shoes danced by himself, edging closer to the group until his father gently pulled him back. He turned out to be Andre Cook, 1 1/2, and his father was Richard Cook, a drummer for the Broadway show "Grease" and husband of Alaina Reed.

"Oh yes, he's seen his mother on TV. He held out his arms to the set. The next time he saw her, she herself was in the room with him. He hasn't figured that one out yet. He's still working on it."

Now Bob was asking questions.He turned to Prudence, 5. "What have you learned on 'Sesame Street?'" he asked. The cameras zoomed in.

"Big Bird," Prudence replied cryptically, "And I can count to 10." Whereupon she counted to 10. Someone else then counted to 11, and someone else counted to 20.

A cameraman with a sticker on his camera that said "PM Magazine, WDVM Channel 9" got into his own news story and asked a kid, "Do you watch PM Magazine?"

"I watch Channel 20," the kid said. Everyone laughed.

"That figures," the cameraman muttered.

The set and Muppets are on loan through Labor Day, since the show is down for the summer, explained Carl. H Scheele, curator of Community Life.

Some of the costumes and artifacts have been given to the museum, and the exhibit also will feature videotape highlights from the show's 10 seasons.

"We're traditionally interested in the history of education," Scheele said. "It's a core activity, in fact."

Compilation firms from the show will be screened at the museum June 6 and 7, and a special workshop for area teachers of the very youn ng on June 14.

In another room, Sesame founder Joan Ganz Cooney was giving interviews to an endless succession of people with pads and recorders.

"It blows my mind that we're going into a museum," she said. "We don't consider ourselves old. We're only 10 and we're changing all the time."

Just the same, she is happy to be recognized by the Institution, since it appears to indicate that the intellectual establishment is at long last admitting the fact of television in American life.

"The show has slowed down a lot since the early days (when some critics accused it of being too exciting to be educational).I don't know if the intellectuals have been watching it recently. When we started, it was th era of 'Laugh-In,' and everthing on Tv was kind of fast-paced, but as the pace has slowed down we've slowed down too. We keep in touch with what's happening."

Constant field research has also helped bring steady changes in the slow, she added. It learned, for instance, that children's attention spans were much longer than at first believed.

"And we're doing more than counting and the alphabet. We examine emotional situations, subling rivalry, things like that. We work with retarded children, and the deaf, and we go into health and safety and nutrition."

Over the years, more than 100 independent film makers have worked for "Sesame Street," which may have something to do with the recent Whitney museum retrospective film on the show.

But it was time to go. Other interviews were circling Cooney and looking restless. Besides, Barkly the Dog was expected on the set any minute. CAPTION: Picture, 'Sesame Street's' Bob McGrath and fans; by Harry Naltchayan-The Washington Post