Names that feel trendy, and end up feeling dated, tend to be one that come out of nowhere and become very popular, very quickly. They’re usually names that have seen very little use in the past, until they suddenly spike in popularity. Many of these had never even been in the top 1000 until they became popular all of a sudden. They’re often newly invented names, or names/words that have never really been used as names before. Many are tied to a pop cultural icon, like Marilyn which was trendy/at its most popular at the time Marilyn Monroe was a big star.
Names that I think are trendy now/will become dated later:
Aria
Luna
Mila
Isla maybe? (especially in the US… it’s got a bit more of a history of use elsewhere)
Emilia (in the US at least, same as Isla)
Eloise
Lila
Thea
Evangeline
Spencer, Blair, Blake, James as a middle, other boys’ names on girls - especially those influenced by celebrities or characters
Made-up names that combine random sounds and add -lyn to the end… Jaylyn, Locklyn, etc.
- ayden names (maybe not Aidan/Aiden itself though, which has more history of use)
Nevaeh… already becoming a bit passé
Harper, Piper, Hadley, other occupational and/or surname names
I think classic names are less likely to become or feel dated, because they’ve been in use so much longer, and even if they were more popular in a certain generation than another, they’ve gotten enough use in any generation that a person with that name could conceivably be any age. They’re also more likely to become classics/be seen as classic over a long period of time even if they’ve had peaks and dips in their popularity. These peaks and dips also tend to last a number of decades/generations rather than just one generation, long enough you couldn’t really guess the person’s age based on that. They don’t tend to be trendy- i.e. very popular for one generation and rarely heard before or after that.
Take Catherine or Sarah, or Peter for boys, for instance- they haven’t been as perennial as Elizabeth or William in their popularity, but still aren’t associated with a particular generation. They aren’t all that popular currently, but no one would be surprised to meet a baby Catherine or Sarah or Peter in 2018 or feel like it’s dated. It’d be more surprising and unexpected to hear it on a child today, but not dated/feeling out of place era-wise. Think listening to the Beatles in 2018, rather than wearing bell-bottoms in 2018. (Not a perfect analogy, I know!)
Personally, I think Emma and Sophia (and Charlotte, Isabella and other names with a long history of use that are currently popular) will be in that category too.
And then there’s the vintage revival names- names that were fairly popular/got a fair bit of use a hundred or more years ago, dropped off or way down the charts, but are now popular or in more common use again, like Hazel, Frances or Florence. If you heard these names now you would probably imagine a young girl, as it was previously popular so long ago that these generations have passed away (think Florence Nightingale or Frances Hodgson Burnett), but they’re not new or flash-in-the-pan names.
These types of names tend to feel old-fashioned rather than dated (which to me implies it was a flash in the pan a few decades ago). If you’d come across a Hazel, Frances or Florence 20 years ago you’d probably have thought old-fashioned (previously popular) rather than dated (previously trendy).
These are my very long, rambling thoughts on it anyways 