Do they? Let's check your source.
> To investigate this hypothesis, participants aged 20–34 perform a concentration and attention test in the presence and absence of a smartphone. The results of the conducted experiment imply that the mere presence of a smartphone results in lower cognitive performance, which supports the hypothesis of the smartphone presence using limited cognitive resources.
So, no. The presence of your smartphone on the desk in front of you is distracting, but that distraction goes away if you remove the smartphone. That's not "cognitive decline."
> Sorry, that's a narrative argument
No, that's me pointing out a competing plausible hypothesis. I'm not saying Covid is necessarily responsible for your anecdotal incidents; I'm saying that until you can prove Covid wasn't responsible, you have no standing to state conclusively that phones were.
It's destroying their ability to experience reality as paths, free navigation, vicarious trial and error, all of this is fundamental to memory consolidation: the brain's fundamental unit: action-syntax in memory, is built from non-screen topological integrations of landmark and allocentric experiences. Phones destroy this.
https://pmc.ncbi.nlm.nih.gov/articles/PMC6059409/
https://www.nature.com/articles/s41598-022-20922-0
https://onlinelibrary.wiley.com/doi/10.1002/brb3.70656
https://www.mdpi.com/2254-9625/15/6/98
https://pubmed.ncbi.nlm.nih.gov/40255102/
https://link.springer.com/article/10.1007/s00246-025-03862-0
https://www.mdpi.com/2227-9067/12/4/503
https://pubmed.ncbi.nlm.nih.gov/40172268/
https://pubmed.ncbi.nlm.nih.gov/40173157/
https://www.nature.com/articles/s41390-025-04024-x
https://www.sciencedirect.com/science/article/abs/pii/S00223...
“A growing body of evidence has found that children’s brains can structurally and functionally change due to prolonged media multitasking, such as diminished gray matter in the prefrontal cortex, where attentional control and complex decision making abilities reside, among other really important skills, like the development of empathy and understanding nonverbal social communication,”
Other studies discuss distraction, cyberbullying, bad diet/poor exercise, toddlers' sensory processing abilities, anxiety, and lost sleep. None of this covers stupidity, and much of it is not about teens.
The most interesting study you cited finds structural brain differences in preschool-aged children who spent more time on screens. It's still a stretch to make claims about the intelligence of teenagers based on the fact that babies who spend too much time on screens are, at age 3, less developed.
There's also a magnitude problem. Even if we assume smartphones do have some cognitive effect on teens, how can we know it's the only or largest one? You can't attribute anecdotes about kids being dumb to the presence of smartphones.
