I'm not talking about having a close personal relationship with your hand here but writing in a way that makes your writing seem old-fashioned. Most writers are savvy enough not to use transient slang in their stories, but there are other, more subtle ways in which writers betray their age. For example, I have a friend whose "tell" is that she knows Yale University didn't start admitting women until 1970 because she entered college in 1969 and wanted to go to Yale. But if she mentions that, it immediately tells everyone that she's 61.
I have another friend who frequently makes reference to obscure movies that came out decades ago, and also makes extremely subtle jokes referencing vintage. That would simply mark him as eccentric but then he attempts to explain the jokes while laughing heartily and it's just truly painful.
So there are ways to avoid the obvious things. But ... sometimes even when you're careful, you're blindsided.
I have a client who wanted his script read by one of my subcontractors, a smart guy in his early 20s. The reader liked the script a lot but was puzzled by one thing. The main character's backstory included a reference to the "Oklahoma City Bombing" with no other explanation.
"The writer does not explain what happened to the protagonist's family," the reader wrote in his report.
He'd never heard of the Oklahoma City Bombing.
He was six years old when it happened.
The writer--like myself--just assumed that everyone would understand his reference.
It's easy to say -- "but everyone knows about the Oklahoma City Bombng," but my smart, young sub-contractor did not.
My client rewrote the script.