Don’t know, but you’ve got my vote!
My immediatethought was that my aging estimation is absolute crap,so I used that first second to decide to try to think of someone in their teens, and the second second came up with Ariana.
As posted above, it was going to take me more than 2 seconds to remember my own name. I did think Taylor Swift, but I needed to google her age and that would be cheating, and besides was going to take more than 2 seconds – and Google says she’s now 30 years old. So anyhow, no way.
Thought of Miley, but she’s been “around” so long, I assumed she was 30 something.
I managed to think of Taylor Swift, dismiss the idea as unfair because she hasn’t sold umpteen-bajillion albums by being “hot” and settle on Kate Upton who legitimately HAS made her career out of her looks in under two seconds.
Now I need to lie down.
Edit: That’s not dismiss anyone else’s answer; just my own frantic reasoning.
Margot Robbie
Now he’s known as an actor who looks like a rawhide chew.
Thought of a couple in their early 30s, who were in their mid-20s when I first heard of them. Does that count? 
But you’re wrong. Cyrus isn’t known primarily for her sexiness at all. She started as a child actress, ffs. And she’s a good singer. Your other names are vaguely familiar.
Jennifer Lawrence–stumbled on her ad for Chanel 5. Don’t know if it was photo shopped or not, but she is evidently the perfectly formed female of the present time in Western culture.
Doesn’t that mean that most stars who were known for their sex appeal are also ruled out, because they had something else going on for them? Not just now, but in general. Like, say, Brad Pitt, definitely known for his looks and attractiveness in his twenties, but was also a very good actor. Even Marilyn Monroe was actually a really good actress especially in comic roles.
The younger Kardashians probably count. Maybe that’s the sort of person we were supposed to think of. Paris Hilton would have been the same fifteen to twenty years ago. There aren’t many famous supermodels at the moment, but I guess Cara Delevigne counts. I’d say it’s fairly unusual to be known only for your looks, though, in any generation, so it would be hard to think of anyone that didn’t also do something else other than look pretty.
Hanna Montana, dude.
But really, she does have an incredible voice. She is very pretty but that tongue thing and naked phase were just weird, not sexy.
No, you’re wrong. The OP didn’t say “primarily”; it said “largely”. I disagree that a young woman who has already achieved iconic status several times including for twerking her ass into a guys crotch on a national TV broadcast and for appearing nearly naked swinging on a giant wrecking ball isn’t “largely known for his or hers good looks or sexiness.”
The first person I thought of was Jennifer Lawrence, 29. I had to think for a few moments but then Sophie Turner jumped to mind.
However, they both fail the test in the OP once I reread it; it said PRIMARILY for their looks. Lawrence and Turner are both beautiful but are primarily known for their acting accomplishments - Lawrence for being an exceptional actress, Turner for being in a ludicrously famous TV show. Taken literally, the OP would exclude most actors and singers; I’d have to think of a model or social media personality.
Nope, just checked: he’s 30.
Man, the twenties got a lot younger while I wasn’t looking.
Most young’uns I eventually thought of (long after two *hundred *seconds…) were in their thirties.
A good follow-up question would be *“And do you care about any of them?” *Also a Nope for me.
Taylor Swift
The OP says “largely”.
And I think it can be argued that neither would have got the chance to act had they not met a high standard of good looks.
Someone like Megan Fox would count as an actress who is, nevertheless, largely known for her looks. She’s aged out of the OP now (33) but, during her heyday, I doubt many people went to see a movie based on Fox’s acting chops.
Nope. I’m turning 40 this year and the first person that came to my mind was 32.
I’m not sure what this says about me, but that’s the first person who popped into my head too. My daughter, now an adult, used to be crazy about him. Maybe she still is?
I saw him a few weeks back on the Ellen DeGeneres Show and thought, “Oh, he’s matured into a handsome young man.” I also saw him in the news a few weeks before that playing pick-up hockey with members of the Toronto Maple Leafs. The kid can play hockey very well!