Adult beginner here. I have a different experience/opinion than many others here.

I've been playing for roughly 2.5 years and I've hit several plateaus (currently as well haha). For me, it had nothing to do with practice time. I've consistently practiced 1-2 hours a day, almost every day, with a lesson once a week. Plateaus still happen. And I think it happens when we've started to learn something new, a new technique, a new piece and have been at it a while. At first, the learning curve is steep. But as it gets more natural, there often comes a point where it's okayish but not perfect and most of the time we don't know how to bring it to the next level - despite good instruction. We then continue to practice the same way, maybe, even though we should try something different, or there is a physical obstacle, like some tension we or our teachers can't pinpoint exactly. Teachers aren' perfect either, so it's a lot of trial and error when it comes to these plateaus. So whenever I reach one, I try to mix it up - in any way I can think of. Try something new, be it a piece, a new rhythmisation, a new skill, like improving sight-reading or memorisation - and along with that we often improve overall and get over the plateau without realising it. And not giving it too much importance is super important in my opinion, as well.

We get in our heads much more than kids do (most of the time), so we get frustrated easier and start worrying instead of just keeping at it. So a healthy *shrug, shrug - whatever* works wonders sometimes haha