Personally, I don't think that is the primary reason adults have a hard time learning languages. Neural plasticity isn't the biggest reason either, IMO.
Rather, the biggest showstopper is that adults tend to always be busy, and fall back to their native languages to get the busy stuff done, and only end up practicing their new language during positive interactions when they have time. Which is really only a very small fraction of their life.
For example, I routinely see people in cross-cultural relationships try to learn each others' languages but fail miserably at learning when they get into disagreements and have that disagreement in English instead of the language either person is trying to learn. And thus, they don't learn. In order to truly learn a language to native fluency you need to be forced to use it for every common life situation, not just the occasional positive interactions. Children are more or less forced to be using the new language 100% of the time.
Rather, the biggest showstopper is that adults tend to always be busy, and fall back to their native languages to get the busy stuff done, and only end up practicing their new language during positive interactions when they have time. Which is really only a very small fraction of their life.
For example, I routinely see people in cross-cultural relationships try to learn each others' languages but fail miserably at learning when they get into disagreements and have that disagreement in English instead of the language either person is trying to learn. And thus, they don't learn. In order to truly learn a language to native fluency you need to be forced to use it for every common life situation, not just the occasional positive interactions. Children are more or less forced to be using the new language 100% of the time.