If Hawaii wants to raise a generation of Peter Pans, they’re on the right path.

Gov. David Ige, a Democrat, signed a bill Friday that bans smoking for those younger than 21. Teens and 21-year-olds can vote and can join the military but they won’t be able to light up in the Aloha State.

The move, Ige said in a statement, “will help reduce tobacco use among our youth and increase the likelihood that our keiki [Hawaiian for “kids”] will grow up to be tobacco-free.”

Will it? The facts provided by the governor suggest otherwise. According to Ige’s office, 86 percent of adult smokers in Hawaii began smoking before they were 21.

Yet Ige’s office also admits that among those 86 percent, only about a third started when 18 to 20. In other words, two-thirds of the 86 percent started smoking when it was still illegal for them to do so. (The legal smoking age in Hawaii was previously 18, although one county made it 21 last year.)

Which suggests that would-be smokers aren’t really too concerned about whether what they’re doing is legal or not—and that the ban won’t necessarily have much, if any, effect on how many Hawaiians end up smoking.

But here’s what it does do: It sends a signal to young adults that you’re not a Real Adult until you’re 21 or older, that you can’t be trusted to have enough maturity and common sense to make decisions about your health at 18, 19 or 20.

Is that the message we want to send young adults?

I know: Modern science shows the brain isn’t fully developed until sometime in your 20s.

But as we’ve seen throughout human history, people, brains fully developed or not, often rise to the occasion and become mature because they have to be.

If that wasn’t the case, no one would pass college (after all, there are often no parents nearby to force students to study), no young soldiers would honorably serve, no employees under 21 would diligently and intelligently work.

Yet, day after day, young adults somehow defy their not-totally-developed brains and do all of those things. They show an ability to delay gratification, prioritize goals and make smart decisions.

So why don’t we trust they will do the same about cigarettes?

Sure, some young adults, like many middle-aged and older adults, will opt for the pleasures of smoking over the health benefits of refraining. (And let’s not forget that idling away your life playing video games or hurting your health by eating too much junk food are both legal at any age.) And that’s OK.

Because here’s what worse than smoking: staying a child.

You can’t grow up if you don’t have any control over what you do. And it shouldn’t be that Hawaii bans those who choose to abide by the laws from making their own decisions about smoking until they’re 21. That’s encouraging immaturity.

Like the generations before them, millennials deserve a chance to make their own decisions—and to make them at 18, not 21.

Unfortunately, Hawaii may just be at the start of a trend. Washington and California have also considered hiking the legal smoking age to 21.

But before baby boomers and others launch more rants about how millennials aren’t growing up and showing maturity, think about whether it’s because they refuse to grow up—or because no one trusts them enough to allow them to make adult decisions.