Growth spurts: They're the enemy of the family sitcom. As the child actors get older, and taller, and more introspective, and less cute, the show loses its early innocence and charm. Suddenly babies are popping out to fill the youth void, cousins are visiting for extended vacations, and the show's original formula is woefully compromised.
''Malcolm in the Middle," which leaves the air tomorrow night at 8:30 on Channel 25, is a case in point. The show began as the affectionately cartoonish portrait of a financially struggling suburban family. There were four rough-and-tumble boys, an ineffectual dad, and a drill sergeant of a mother named Lois, played with kooky brilliance by Jane Kaczmarek.
The out-of-control kids turned the messy house into a sort of life-size game of Mousetrap, while Mom freaked out. Malcolm (Frankie Muniz) was the sane son in the middle of the madness, a bright kid who'd speak directly to the camera about life, love, and his predicaments.
Seven years later, Muniz is 20 and far from the sweet little creature of the show's early years. While the family now has five sons, after Lois gave birth in 2002, the focus still falls on Malcolm, as well as on his older brother Reese (Justin Berfield), a thick-headed troublemaker who also is no longer cute. The writing is as lively as ever, and the single-camera production remains freshly off-kilter and surrealistic, but the main characters have aged out of the show's system. They look like young adults who've been dragged to Disney World by their parents.
And so it's time for ''Malcolm" to say farewell, which it does with sweet insanity. In the episode, Malcolm is graduating high school and Reese is hoping to land a full-time job as a janitor. Cloris Leachman is on hand as the kids' hard-drinking and bigoted grandmother, who gives Reese some advice that turns out to be disastrous on Malcolm's graduation day. Meanwhile, Dad (the effusively wacky Bryan Cranston) turns to a loan shark to pay for Malcolm's Harvard tuition.
The half-hour sails by, with none of the grandstanding and schmaltz many finales rely on to make an ending profound. Creator Linwood Boomer finishes his series with the same flip warmth he began with, as the strung-out family -- whose name, Wilkerson, was used only once -- stand together. No matter how crazy things got during the past seven years, and they got pretty darn crazy, Malcolm, his parents, and his brothers were inextricably bound together through thick, thin, blackmail, lies, fires, and thefts.