Thank you for including sources! I dispute them, but I value the fact that you included them. Here’s why I dispute them:

• The first one is from the University of Chicago, which is one of the most left-leaning institutions in the nation. They took their data from the Michigan Youth-Parent Socialization Panel Study. A different study in a more typical part of the US, using the same methodology, would yield quite different results.
• The second one is an editorial from the Guardian, a left-leaning publication. That’s not to say it’s entirely false, but it’s certainly biased. Also millennials are still too young to observe this effect very well, and the articles all about millennials.

And a couple more reasons:

• This is entirely anecdotal, but personally, I have become way more conservative with age, having been a drugged out punk youth, and gradually transforming into a Christian conservative adult. I know quite a few people who changed similarly, and I know nobody who changed in reverse. Anecdotal, I know.
• I think having Trump in office generally delayed the effect of young people becoming more conservative. Not that Trump is all that conservative. But few young leftists looked at Trump and said “I like that guy.” And by “few” I mean yes, there really were a few, but it was not close to the majority.

By what measure or definition are you using the word “conservative” here?

In the broad American sense: someone who wants to preserve traditional Judeo-Christian values, with either a strong belief in “liberty or death”, a strong support for the police, or — in cases where cognitive dissonance runs strong — both.

What is the cutoff for an “adult”, is that the same as being able to vote or are you drawing an arbitrary line at an older age?

Your guess is as good as mine. It’s subjective.

Surely you are not suggesting that “almost every adult” is a GOP voter, because even Trump would not be so brazen as to pretend there are no Democrats at all!

Ha, do not underestimate Trump! Lol. I do think if you look at voter affiliation by age bracket, there’s a certain age threshold above which 50% or more voters are right of center.

I completely reject the equation of maturity with “conservative and traditional”.

I respect your rejection of that equivalence. I am certainly biased, as I believe we all are. That assertion just reflects my experience. Take it or leave it.