Are you a real adult if you're still on your parents' family phone plan?

• Millennials, some now into their 40s, are still on decades-old family phone plans.
• Sure, this can seem like arrested development or mooching. But really? It's just smart.
• There's no reason not to save a few bucks on a phone plan.

Before you yell at me, let me first say that I am not, and have never been, on my parents' cellphone plan. (I didn't get a cellphone until I was already an adult.) But I've long been jealous of my friends who are still on their parents' plans — it just makes good sense!

Yahoo News recently asked whether staying on your parents' phone plan as a 40-year-old makes you "a harmless mooch or a generational failure?"

The reporter, Fortesa Latifi, admits that she and her husband were still on their parents' plans until recently, and that many others are like her, some even with children of their own, and quite a few feel embarrassed about it.

There are significant savings to be had by joining a family plan. For example, right now, at T-Mobile, its unlimited talk, text, and data plan costs $85 for an individual plan. For a family of four, the same plan is around $42 per person.

Why are family plans so much cheaper per line? It's not that there are a lot more costs to operate cell service if a phone number isn't connected to a family plan. It's all about how advantageous it is for the carrier to sell family plans.

For one thing, if you're part of a family plan, you're less likely to shop around and switch carriers. It's also easier on the carrier's customer service: They only have to mail bills, process credit cards each month, and all that jazz for one person instead of several. (Verizon and T-Mobile didn't immediately respond to a request for comment on their pricing.)

Last year, AT&T added a new feature that makes it easier to automatically split the bill for people who share a friends-and-family account. The person whose name is on the bill is still ultimately responsible for the full amount, so enter into this kind of arrangement only with people you really trust.

AT&T pointed me to a news story published last year that quoted an exec saying 85% of their customers were on a multi-line plan. Think about that — that means if you actually are one of the suckers who is paying for a single line, you're in the vast minority.

There's no honor in paying more to have the bill in your own name — you're just paying more for the same services. Does your dignity and independence win out here, or does T-Mobile? Hmm?

Does having your own cellphone line make you an adult?

AT&T released its own study (so take it with a grain of salt) that said that 76% of Americans think that coming off a parent's cellphone plan is one of the "ultimate signs" of becoming an adult.

Sure, at first glance, this seems like a rite of passage into financial independence from your parents.

Is it a smart financial choice?

Consider that the T-Mobile plan — even if you paid back your mom each month for your portion of the phone bill, you'd be saving about $42.50 a month compared to the same service on an individual plan. That's $5,100 over a decade if you did it from age 22 to 32.

In fact, I'd say that part of becoming an adult is being smart about spending habits and money. And sticking to a family plan is the obviously wise choice.

If you choose to remove yourself from a family plan, you're just giving the cellphone carriers twice as much — and I see little glory or pride in that.

Look, of course, this all depends on your relationship with your family. You may not want to have this financial tie to them, and you may be in a better financial situation than your parents. But bundling phone lines with other people, whether they're your family or just some friends, makes a lot of financial sense.

Millennials, it's time to take pride in one smart financial decision that our generation is making. Embrace it! Be proud to be on a family plan!