For young couples, major relationship milestones used to look something like a borrowed letterman jacket or a shiny key to an apartment — a show of trust and commitment, and a means to say, what's mine is yours.

Younger millennials and Gen Z teens, more tech-savvy and less privacy-keen than generations before, have kept the sentiment, but replaced lending their letterman jacket with swapping biometric passwords like a fingerprint or facial recognition ID to access their partner's phone.

As smartphones like those made by Apple and Google increasingly move toward biometric access, younger users lean on the convenience of a single tap or look, over traditional passcodes. And with the option to store multiple fingerprints or facial appearances, couples are lending a finger for easing unlocking of their significant other's device.

"I think that inherently, people desire to share themselves and to be known. Sharing your phone fingerprints demonstrates trust between two people, and that you are OK with being known by that person, and that they're OK with you knowing them too," said Emma Clarke, a 24-year-old living in New York City who's previously swapped fingerprint biometrics with a boyfriend to access his phone.

"People like having a special connection with someone that others don't have and showing it off. It's the modern version of putting your boyfriend in your top friends on MySpace," Clarke said.

Most of the young couples I talked to noted the convenience of having one-tap access to a significant other's phone. Switch the song while she's driving, check the recipe while his hands are busy cooking. Her phone has apps his doesn't.

Still, sharing a numerical code could grant access for those occasions, which is what several 30-plus-year-old couples told me. Several Gen Xers or Baby Boomers I asked hadn't ever considered swapping fingerprints, and weren't easily convinced by a list of conveniences.

Storing a partner's biometric data seems to be a uniquely younger compulsion, and likely tied to the typical millennial nonchalance around cybersecurity that comes with a digital childhood and regular headlines of data breaches. Younger generations are more used to, even more comfortable with, the risk of having sensitive information stolen, so sharing an otherwise strong security metric doesn't register as risky.