Some notes, coming from my recent experience as a guy in his 50s learning Spanish. I'm trying to generalize what I've learnt about learning; I'm curious to know others' experiences.
Phase 0. The baby: Copying sounds, like a baby does. This *can* be accompanied by reading the alphabetic representation of those sounds, although that is the easy part; the hard part is learning how the sounds work. Notice this should be at least majority target language (i.e. you can have translations of standard phrases for meaning, but your *work* here is to copy the sounds).
Phase 1. The struggle: practicing rules, playing games, and achieving understanding with limited knowledge. This phase is often accompanied with significant usage of mother tongue (i.e. not the target language), which is to be minimized if possible.
Phase 2. The snowball: can now enjoy consuming what interests you (e.g. films, podcasts, TV). Continued osmosis of more patterns. At this point you should be operating 90%+ in the target language (translations are only a last resort, you still use them but you avoid them).
Phase 3. The totality: the language becomes part of (or more usually, all of) your everyday life experience. This is the normal path to fluency.
It's interesting to observe (IMO) that your entire language learning process can actually be via only *ONE* of these phases, and it *can* lead to fluency, as crazy as it seems.
There are stories of adults learning an entire language through only phase 0 (an American who lived in Thailand (when there were few foreigners; perhaps 1950) for several years without speaking a single word, but eventually spoke fluently with a perfect accent because of it). This seems to be extremely uncommon, except for the mother tongue acquisition of course.
There are also adults who prefer to go straight to Phase 3, often speaking terribly badly at first, and definitely picking up ossified errors in pronunciation and grammar, but for a certain type of person this works really well, *if* they have the kind of personality and lifestyle that allows them to integrate into a society while learning (or sometimes, they have to just force it; they need to work in a foreign country).
There are those who use Phase 2 for a very long time, for this type of person the language is more of a hobby, albeit a high level one: they simply consume but over time do so at a near-fluency level (good example, many people outside of Anglo countries watch US TV and movies their entire life, or something specialist like anime, and develop full listening fluency, though it may be somewhat specialized). Such a person will usually be competent at using the language more generally, and can eventually be properly fluent.
Phase 1 is probably the least likely (even less than Phase 0! , though I guess that's debatable) to result in full fluency. This can be seen as the "academic" approach: study the language, and practice it within defined sets of exercises. The biggest problem with this (and in a different way, Phase 0) is that it's hard to keep motivation for the literally many years that an adult would need, to reach fluency with this approach.
As you've probably gleaned, in this framing, the reason adults usually end up reaching fluency or at least competence via Phase 2 or 3 is because it's only there that you have easy and full *motivation* to keep working on it. That's why the crossover from Phase 1 to Phase 2 is so crucial; I think a lot of failed language learning fails because of not choosing to cross that threshold, at which you can start to simply enjoy consuming, early enough.
This is why I have become a big advocate of the acquire-through-consuming method, over the traditional "learning" methods which seem to be very ineffective. The best way to get over the hump of Phase 1, in my experience, is to spend *little or no time on grammar exercises or any rote learning*, and perhaps 90% of your time on finding ways to consume content in the language, even if your level of understanding is as little as 50% or less. Best place to start is TV shows with target language subtitles, since it has so many cues to understanding, and honestly, keep this as the bedrock of your learning process all the way to the "end of Phase 2", if that makes any sense. Which for an adult who's older, could be years, not months.
Also this is why I would *never* recommend e.g. Duolingo. This is for my money just a terrible approach, it's turbo-charged Phase 1 to simply play a game of learning words over and over. Probably a controversial opinion.
And by the way (I always make this recommendation but not sure if anyone ever really took me up on it), I *strongly* recommend Rosetta Stone audio files for Phase 0. It worked perfectly for me with Spanish.
