Why does my adult child argue with everything I say?

Because the arrangement changed and neither of you announced it.

Somewhere in their twenties your child ended up knowing a world you don't: their field, a technology, a body of reading, some way of thinking about things you only half follow. Having found one place where your account was out of date, they start checking the others. From their chair that's ordinary diligence. From yours it arrives as a person who used to listen and now contests everything.

The reason you didn't see it coming is structural. Vern Bengtson and Joseph Kuypers gave the pattern a name in 1971, the developmental stake: what each generation has riding on the relationship bends what it notices about it. Children read the distance as wider than it is. Parents read it as narrower. And nobody who has held the informed seat for three decades is keeping an eye on the week it quietly stopped being theirs by default.

Sometimes, and mostly not.

There's a real difference between being argued with and being talked down to, and it's audible if you listen for what's being disputed. Someone challenging your claim is treating you as a person who makes claims. Someone rolling their eyes at the fact that you have one is doing something else. The first is a conversation you're both still in. The second is worth naming directly, in the moment, as its own subject.

The trap is that the first reliably gets answered as though it were the second. You raise your voice to the level disrespect would deserve, and every conclusion they have been quietly drafting about you gets its confirmation. That exchange is where the wedge actually goes in, and it's usually about tone on both sides rather than about whatever you were disagreeing over.

Then say so. Being demoted from oracle to person doesn't oblige you to agree with anybody.

Two things are getting confused here, and separating them costs nothing. One is standing: whether your view arrives with the automatic benefit of the doubt it used to have. That has genuinely changed and won't be changing back. The other is whether you're right, which the first has no bearing on at all.

What helps is making the disagreement about the subject rather than the seating. I think that's wrong, and here's why travels. I've been doing this since before you were born is a claim about rank, and rank is the exact thing under negotiation, so it converts a solvable disagreement into the unsolvable one. A lifetime of noticing how people actually behave is evidence. It just has to be offered as evidence rather than as seniority.

Less than you'd think, and earlier than feels natural. The whole move is one sentence that names the change out loud before it hardens into a fight about respect.

Related: How to have an adult relationship with your grown child has the four moves in order, including the sentence and what to do when it goes badly the first time. The longer argument, from both chairs, is in The Year You Became a Person to Them.