That "brain matures at 25" bs is a myth that was caused by a study in brain development losing funding when its subjects (that it follwed from birth) were 25.

Concluding brain development stops there is like assuming the road ends at any point where you have stopped following it.

That's because of the "language tiers".

People don't usually learn languages for fun, at least not to a point where they can actually speak it fluently. They learn it because they have an use for it. If you learn a language without having an use for it, you lose it quite quickly.

The highest tier language is the worldwide lingua franca: English. You learn English to talk to anyone, not to talk to English native speakers. For example, my company (a central European one) uses English as the work language. We don't have a single English native speaker on the team. But if I want to talk to a colleague from Rumania, Egypt, Spain or the Netherlands I will talk English with them.

The next tier is the regional lingua franca. That's e.g. Spanish, French, German, Mandarin, Russian or Arabic (and likely a few others, I don't know the whole world). These languages are spoken in certain regions and can be used to communicate with people from neighbouring countries. You can get around with e.g. German in Hungary, because most Hungarians learn German. It's also sometimes necessary since TV, books or other media might not be available in the local language. For example, a lot of Albanians speak Italian, because TV shows and movies are rarely translated into Albanian and instead broadcast in Italian. (Also, since Italy was so close, many people watched Italian TV while Albania had communism.)

The lowest tier are local languages. These are languages that are only spoken in their own country. For example: Rumanian, Serbian, Hungarian, Welsh, Gaelic, Dutch and so on. People speak these languages because they live in that country. For someone who doesn't live in that country, there's rarely any major benefit to learning these languages.

In general, people only really learn to speak languages that are on the same tier or higher.

If you live in Albania, you learn Albanian as a child, then probably add Italian to understand TV. In school you will learn English and once you go online you will use it. You might also learn Russian to be able to communicate with people in nearby countries and if you are from the muslim part of Albania you might also learn Arabic.

If you live in Germany, you'd just learn German and English. No need for any other languages. If you spend some significant time in France, Spain or Italy, you might pick up one of these languages.

If you live in the US or GB, you start with English, and there's hardly any point to learn anything else. By default you can already communicate with everyone, read everything on the internet and watch all TV shows and movies (pretty much everything is translated into English, if it isn't even refilmed in English). If you try to learn another language and try to use it with native speakers of said language, chances are pretty high they just switch over to English.

In my experience when I lived in Holland, compared to me my friends and colleagues from English-speaking countries had the additional problems in trying to learn Dutch that people would tend to switch to English when they heard them speak in Dutch (probably because they picked up from their accent that they were native English speakers) plus their own fallback when they had trouble expressing themselves or understanding others in Dutch was the "lowest energy" language of all - their native one.

Meanwhile me - being a native Portuguese speaker - suffered a lot less from the "Dutch people switching to English when faced with my crap Dutch language skills" early on problem (probably because from my accent they couldn't be sure that I actually spoke English and they themselves did not speak Portuguese) and my fallback language when my Dutch skills weren't sufficient was just a different foreign language.

So some of my British colleagues over there who had lived there for almost 20 years still spoke only barelly passable Dutch whilst I powered through in about 5 years from zero to the level of Dutch being maybe my second best foreign language, and it would've been faster if I didn't mostly work in English-speaking environments (the leap in progression when I actually ended up in a work environment were the working language was Dutch was amazing, though keeping up was a massive headache during the first 3 or 4 months).

That said, some other of my British colleagues did speak good Dutch, so really trying hard and persisting worked for them too (an interesting trick was when a Dutch person switched to English on you, just keeping on speaking in Dutch).

Good job. English is a very hard language that barely uses logic.

I am from Portugal - which is a very peripheral region in Europe, bordering only Spain - but do speak several European languages, and one of my most interesting experiences in that sense you describe was in a train in Austria on my way to a ski resort, an intercity train (so, not even a long-distance "international" train) which was coming from a city in Germany on its way to a city in Switzerland just making its way up the Austrian-Alps valleys, and were I happened to sit across from two guys, one Austrian and one French, and we stroke up a conversation.

So it turns out the French guy was a surf promoter, who actually would often go to Ericeira in Portugal (were at a certain time in the year there are some of the largest tube waves in the World, so once it was "discovered" it became a bit of a Surf Meca) only he didnt spoke Portuguese, but he did spoke Spanish.

So what followed for a bit over an hour was a conversation floating from language to language, as we tended to go at it in French and Spanish but would switch to German to include the Austrian guy and if German wasn't enough (my German is only passable) we would switch to English since the Austrian guy also spoke it, and then at one point we found out we could both speak some Italian so we both switched to it for a bit, just because we could.

For me, who am from a very peripheral country in Europe, this was the single greatest "multicultural Europe" experience I ever had.

That said, I lived in other European countries than just my homeland and in my experience this kind of thing seems to be likely in places which are in the middle of Europe near a couple of borders and not at all in countries which only border one or two other countries.

Maybe unsolicited advice but I have gotten my Spanish to a decent level, and I'll paste a comment I made a year ago somewhere else below if you want to hear the method I used.