Charlie had a baby girl with Daphne...they named her Diana (after his mother). So she was the youngest Salinger when the show ended. But Owen for sure was the youngest of the original 5!

Diana

Amazingly, this simple question seems to have stumped everyone. Owen is incorrect and "Stewart" was never actually born on the show, so that answer is incorrect also.
Keep trying.

Lester B. Pearson.

It was a small part played by two 4 month old triplets of the same name. The episode was entitled "The Last of the Unplucked Gems" and had what many critics called 'an air of sophistication that was unrivaled by any of the other episodes'.

Many critics also felt that the introduction of this character was the turning point in the show that eventually led to the infamous 'red rave dagger dance' conflict between Neve Campbell and Piper Perabo (Who was originally cast for the role but was replaced at the last minute due to a misdiagnosed brain hemorrhage.)

Anyway, the name you're looking for is Lester B. Pearson or LBP as the taxi driver who delivered him used to say.

Man, that was a great show.

Or were about to, anyway. I think Kirsten was still pregnant in the final episode.

@Kim_Eriksen: I didn't say Owen was Charlie and Kirsten's child. I said that when Charlie and Kirsten had a child, they allowed Owen to name him (in the final episode). He wanted to call the baby "Stewy," but they lengthened it to Stewart Nicholas.

Charlie & Kirsten's kid Owen was the little brother Owen (they adopted him), there weren't 2 different characters named Owen on the show just the one, who was the youngest.

Oh, and then they decided his full name would be "Stewart Nicholas Salinger," but they'd refer to him by his middle name, Nicholas. Julia, ever the scamp, jokes that people will call him "Snick." Off-camera, I'm guessing Stewy Nicholas ran away from home at 15.

The youngest Salinger's name is Owen.

You're either referring to baby brother Owen, or to Charlie and Kirsten's child, who was named (by Owen!) Stewy, thereby foreshadowing another famous TV baby with wacko parents.