The story outlines the results of the General Social Survey, showing that 26% of millennials claim "no religion". [nitpick #1 - this is not the same as atheism or agnosticism (in fact, I wonder how much of this effect can be blamed on the idiotic assertion that Christianity is not a religion, which has come into vogue [I presume as a means of distancing Christianity from criticisms of religion in general, or to take the sting out of the use of analogies between Christianity and other religions. ["Oh yes, what you say about those frog-faced heathens and their religion is true, but since Christianity is not a religion, it doesn't affect me and my special personal relationship with Jesus."]]] Regardless of my nitpick, that's by far the highest value recorded on that question for different generational cohorts.
But if that is not the whole story, what is the rest of the story?
"Some millennials are just graduating from high school, and traditional wisdom (and statistics) show that people grow more religious as they age."
When you see "traditional wisdom" quoted as a source, that should sound the alarm bells. And that sad little "and statistics" clothed in parentheses like camouflage does not increase my confidence, especially since no source is provided. [The linked page for 'The numbers may be deceiving.' is not relevant to belief as people age.]
But if you look at that graph, I think the thing that leaps out to you is that the lines are flat, i.e. people's self-reported affiliation to a particular religion is quite constant (at least in the aggregate).
The graph even helpfully puts a fat dot on the graph to show when each cohort was the same age as the "just graduating from high school" millennials. Gen X hasn't aged much, but it starts at 20% and ends at... 20%. But the Baby Boomers over 40 years have showed a dramatic change from 13% to 13%.
The data doesn't go back far enough to show the older generations at high school age, but as far as we can judge the trend, it shows that as people age from adulthood to senior citizenhood, the proportion claiming no religion actually increases a couple percent in both cases.
(Possibly other studies show something different, or gauge intensity of belief in some way that maybe increases over time, but just looking at the presented data on the topic at issue, I just don't see any "rest of the story" there.)