It's a parent's worst nightmare: A toddler with a milk bottle in one hand and a crayon in the other quietly sits down in front of a $1,500 computer and, in a matter of minutes, figures out how to short out the keyboard, draw whiskers on the mouse and erase the contents of the hard drive.

It's an exaggerated scenario but one that many parents can't help but imagine as their child's interest in computers and technology grows. And it leads to the bigger question: What age is the most appropriate to expose a child to a computer and maybe buy one of her own?

"I think it just depends on the kids," said Laurie Lipper, co-president of The Children's Partnership, a Santa Monica, Calif.-based nonprofit agency that wrote the Parents Guide to the Information Superhighway.

The guide - available online at www.childrenspartnership.org/pub/pbpg.html - notes that parents should avoid treating computer use as a developmental milestone of a child's growth. "There are no 'shoulds' in this arena like 'a child should walk by 15 months,'" the report read.

Instead, a child's contact with the computer should be gradual and the content should be complementary to other learning activities, said Evelyn Dubocq, spokeswoman for Riverdeep, the Fremont, Calif., company that makes educational software under The Learning Company brand.

"If your 3-year-old is advanced enough and you feel that exposure to the computer will be valuable to them, sit down and play with them at the computer," she said. "The computer should not be a babysitter. We emphasize that strongly."

Early exposure to a computer - with an adult "playing along" - can help children with basic motor skills such as hand-eye coordination and can give them the critical thinking skills they'll need to learn better in a classroom setting, she said.

"It starts to teach a child how to learn," Dubocq said. "Children have to be shown how to learn. That's important as they move on to higher grades."

But computers and software shouldn't replace the instruction methods of a classroom teacher, said Randy Hollenkamp, the technology resource teacher for the Evergreen School District in San Jose, Calif.

"Learning at those young ages is very social," he said. "Children get the most benefit from the classroom activities they do. If you visit a kindergarten or first-grade class, you'll hear a lot of acting-out sounds. That's where they're learning how to read. If they sit in front of a computer, they'll add to the knowledge base, but it's not that they learn how to read from it."

Still, computers and technology becoming increasingly important to a child's education as he or she advances in school, Hollenkamp said.

Around the fifth grade, the demands of homework increase and the amount of time a student spends writing reports, researching information and fine-tuning their motor skills on more complex games becomes more valuable.

In some cases, it's when a child reaches that age - about 10 - when many parents start shopping for a second computer for the home.

Dell, which sells more personal computers than any other maker, sponsors education initiatives targeted mostly at middle school students, the level where children start to have in-depth homework assignments that require computer and Web access.

Children in the sixth to eighth grades find the amount of time devoted to school-work jumps - and there may be no time for anyone else in the house to sit down at the computer to help.

Still, the company cautions parents about buying a computer for exclusive use by a preteen, especially if that computer has a connection to the Internet.

"Computers should be in common places in the house so parents can monitor kids' activities," said Dell spokeswoman Jennifer Jones. "It should be in a central location."