Ok, I'm just gonna bulldoze my rep and point something out to the TERF hordes:
Let's say the teenager was going through a phase & exploring an identity but ultimately identifies as cis.

Let's even assume this happens, being absurdly generous, 50-60% of the time.

SO. WHAT?

Suppose, in your caricatured world, that a plurality of trans kids eventually decide that they identify with the gender they were identified as birth! That it was just a phase, a trend!

YOUR CIS KID WILL STILL HAVE A MORE NUANCED IDEA OF THEMSELVES FOR THE EXPERIENCE.

Even IF we assume that some portion of trans identity's surge in adoption is a trend, SO. FUCKING. WHAT?

Yeah, a lot of folks might end up trying it out! Who gives a shit? ELECTIVE IDENTITIES REMAIN VALID.

The only casualty in such a case is your comfort in your assumptions.

Hell I think it's reasonable to assume we are in a transitory phase of mass experimentation with gender identity while society figures out how it's going to make room for diversity beyond a binary cishet norm. Who CARES?! Why do you? Why are you so fucking AFRAID?

It is my personal suspicion that a significant portion of the population identifies as cishet circumstantially - not that they are dysphoric, or not cis, but their gender ID, like other IDs, is not a timeless fixed Platonic self but evolves form material & social possibility.

And even if this is the case, REGARDLESS of whether or not trans identities are Born This Way, elective, or both: Trans IDs are still valid, and the broader exploration and tolerance for them is a net good.

Odds are it probably won't ever fucking AFFECT you anyways!

Society has explored the idea of gender fluidity and transition for probably as long as humans have HAD a notion of gender. It's always interplayed with caste, social sphere, etc, but we've almost always acknowledged it as more fluid than deterministic.

The only reasons I can see for your terror are trauma and propaganda - either hatred of anything that does not fit within your narrow and deterministic boxes, or lies and horror stories designed to feed your war-of-the-sexes fears & enclose you in narrow spaces.

Grow up. Let go.

This does not have to be the center of your fucking lives, any more than any other mass panic over social trends which ultimately still affect a tiny minority.

Just let go. Give people their space and odds are they will give you yours, including for your subjective self-image.