Why do some names date and some don't?

Mclaren37 Elizabeth is still quite popular. More popular than names like Ellie, Lola, Bella, Thea, Emma, Violet, Harper and Amelie.

Timeless names are those which have been consistently popular over a long period of time (or relatively popular), but which didn't shoot up to top 10 popularity within a couple of years and then drop off within the next 5-10 years.

Currently popular names which I think will become new "timeless" names are Olivia, Amelia and Grace for girls. Possibly even Chloe, at least where I live, as it has been popular for 20 years now.

As for boys, Oliver is about the only one in the top 10 (here) currently which I think will be a stayer in the long-term.

Names that are uncommon will not date unless they have a burst in popularity. So you could choose Genevieve and you'll probably be golden, as it's been around forever but has never experienced a rapid rise in popularity and there's no indication that it will in the near future. You wouldn't be able to determine what age a Genevieve was based purely on the name, whereas you could make a fairly accurate deduction for someone named Evie or Isla.

There are a few names that haven't been out of the top 100 names since 1905, so truly timeless names. For boys they include William, James, Charles, George, David and Thomas. For girls I could only find two names, Sarah and Elizabeth. This was from the baby names interactive website.

I think if you know people with the name and they are different ages from young to old, then it probably won't date, or it might but will come back into fashion. Royal names are usually safe. Pet forms can date.

Names that are new tend to date - pop stars, footballers and tv show names spring to mind. Kylie, Rio, Kian, Kayleigh, Beyoncé, Rihanna, Skylar, Shannon, Shanice, Amelie etc. will probably sound dated.

Soap opera names tend to be ones that date. If you think of characters in Corrie or EE, they tend to be popular for the character's age group.

(And Catherine and Elizabeth are very dated to me! Classic or not. Every other woman I know is called Kate or Liz - all of my generation. Very few young people (thank goodness!). I guess we all get tired of our own generation of names.)

I think it's hard to get perspective on what's fashionable when you're in the moment. People have suggested that Henry won't date because it's an old / royal name. But 15 years ago Henry was seen as really quite 'out there' as a baby name (unlike Harry which was uber popular). I worked with a Henry and it was viewed as an almost comical name on a grown man.

Now it's incredibly popular and so it seems more acceptable as a classic. Names that feel classic / vintage now would have been deemed ridiculous a decade ago. I think Henry will date a child to 2016-ish the same way Chris Mike and Dan date guys to the late 70s. They are all still nice solid names, but definitely datable.
I think the only way to come up with a name that doesn't date is to go with something incredible unusual and therefore often not very popular or to current taste.

Like fashion, names that rise in popularity quickly and become super trendy tend to fall out of fashion as people get bored of them, making them sound dated

This ^

Something that is the height of fashion / very 'on trend' / etc in your wardrobe, will then look 'so last year' - er, the next year . Whereas a more classic but less fashionable item - what you might think of as 'stock items' of people's wardrobes, don't 'date' for a decade or so.

Also, what you say in your 2nd post.
It seems names that have been very popular in the last 10 years or so have seemed really "old lady" / "old man" to me (in my 50s) , because they are the names I think of great Aunts and Grandparents having, but of course the generation naming these children, won't remember my elderly relatives, and so the names come back into use. In another 30 years I suspect everyone will be reviving Susan, Karen, Debbie, Kevin, Gary, Paul, Stephen, etc.

Names that are connected to a particular person or event will date very quickly. Like all the women in the 60s called Jackie after Jackie Kennedy. Or all the girls born in the late 80s/early 90s called Kylie.

Like fashion, names that rise in popularity quickly and become super trendy tend to fall out of fashion as people get bored of them, making them sound dated.

That applies to classics too. Stephen, Andrew, Sarah etc all sound a little dated as they were very fashionable in the 70s for example.

I think it's often generational. Names of your own and your parents' generation seem dated to you, whereas by the time you're of an age to have your own children the names that were current in your grandparents' generation have become part of the new zeitgeist again.

Royal names don't seem to date: James, John, Charles, Henry, Catherine, Elizabeth etc ...
With others maybe it's a generational thing - I wouldn't dream of calling my kids any names from my parents generation: janet, Colin etc but by the time my kids are grown up they might see them as fresh and quirky. In turn, is that why "old person" names are popular now: Harry, Archie etc? Just a theory anyway!

I think that names that rise very quickly in popularity are the ones that become dated, while ones that have been used consistently for centuries are seen as classics.