Forget wrinkles. A sure sign we're aging is that Big Bird is turning 35 (his publicist swears he's not a day over 6). Our buddies from TV's favorite hood are feting their milestone birthday with a prime-time special airing April 4 on PBS. Which got us thinking about what we've learned from our plushy pals since their 1969 debut: coping skills when Mr. Hooper died, acceptance of the ambiguously gay, counting to 12. So from us, Sesame Street, a toast. Go ahead. You're old enough.