But many adults spend years immersed in the country and still make many mistakes, have bad pronunciation, and can’t use high level grammar. Some can’t speak the language at all.

There are very specific reasons for that. People who can’t speak the language after many years are getting very little to no comprehensible input. This happens often when people move to a country without knowing the language. Since living like a local would be quite inconvenient for them, they make many little lifestyle choices that end up creating a bubble around them that doesn’t require them to learn the local language. They reside in a part of town where they can get by using English or their own language. They go to stores and restaurants where they know they can communicate with the staff. They hang out around other foreigners and become friends with other foreigners or locals that can speak their language. In practice they get very little input in the local language. When they do get some input from street signs and people’s conversations, their level at the language is so low that they can’t benefit from it, since it’s not comprehensible to them. For people who do learn the language and become good at understanding it but still make many mistakes or have a strong accent, the reason is not lack of input, but rather early output. We explain more about this in the question Should I ever practice speaking? When?. Basically, people who started speaking the language before having received enough input, have entrenched bad "habits” and pronunciation that are very hard to get rid of. In the case of immigrants, they did what they had to do to survive and get by and made the right decision under their circumstances. In your case, you can decide how important it is to you in having near-native grammar, pronunciation, and idiomatic use of vocabulary, or whether you need to make more immediate use of the language, and decide when to start speaking based on that.