Nicholas is 9, Nathaniel is 9, both born the same week. We won’t reveal which one is older. It can be a sensitive subject.

Nicholas lives in Philadelphia, our home to be specific; Nathaniel, in Manhattan. Location, contrary to Realtor claims, isn’t everything, but it can make a difference. Despite these cousins’ unfettered affection for each other, there’s no question who is growing up faster.

Nathaniel listens to rap spiked with language — unprintable here — that he hears on the street every day. The other day, Nicholas asked who Eminem is.

Nathaniel is beyond animation. During a recent visit, the kids had an arduous time selecting a matinee. Nathaniel disdained “Treasure Planet,” which his cousin was panting to see. Nathaniel was excited about “Adam Sandler’s 8 Crazy Nights,” rated PG-13 and studded with profanity. He’d seen the James Bond movie. Twice. Nick asks if he can see “Die Another Day,” which would be his first movie with guns and orange bikinis. Over our dead bodies, we say.

Nine is on the brink. It’s the bridge, often a nice, long one. Awhile ago, Nick started convulsing in laughter after seeing the word “sex” printed on my driver’s license.

We all know where 9-year-olds are going, on to an eternity of double digits. Once they get to that world of guns and explosions and understanding all the epithets in “8 Mile,” there’s no turning back. The innocence has evaporated.

Routinely, we are called upon to make decisions about what Nick is ready for, how wide we want to open the dam’s locks. We agreed to “The Simpsons,” a program we all enjoy though sometimes we wince at the sex and drug allusions. They fly over the head of our daughter, 6 and still marveling at Marge’s tower of azure hair, but it’s clear that, like a tourist with rusty French, Nick is picking up half of what’s being said.

As the gatekeepers on his path to maturity, we’re clearer about shielding him from popular culture than about what to withhold about the world at large. He’s asked about World War II, but we haven’t told him the horrors of the Holocaust. He’s learned about slavery, but not the lynchings of the more recent past.

We decided to let our son see the towers fall, though not the planes crash into them, and worse. Foolishly, wrongly, I told him I thought Osama bin Laden was dead to assuage his fears, and mine. Then, months later, I was torn about telling him about the al-Qaida leader’s audiotape, about the terrorist bombings in Bali and Mombasa.

Our friends’ son, as a kindergartner, sat on a sofa, drinking from a juice box, stroking his cat, watching his favorite cartoon. “You know why I love being 5?” he told them. “Because it’s so easy.”

Five is a piece of cake. Nine is a more complicated torte.

There’s little in life more marvelous than a sweet, still-affectionate 9-year-old, wise enough to be engaged in the world, young enough to enjoy cartoons. His choices, though, are coming at a more rapid clip, like balls in a batting cage. The trick, which I’m still learning, is knowing which ones to swing at and which ones to avoid. For now, it’s clear. He can have Bart. Eminem will have to wait.