I've learned some basics to four or so and a few writing systems post-30's and will keep going. Other two learned in school. The joy and motivation is night and

I've learned some basics to four or so and a few writing systems post-30's and will keep going. Other two learned in school. The joy and motivation is night and day better when you're not in school IMO. But since that's not really data, I'll offer that I'm adjacent to a workplace where people come and go from around the world. Some may stay a few years, some may stay longer. Several adults have learned this massively obscure and quite hostile language (Finnish) while staying here even though they mostly get along with their work in English. I'd say there's no hard constraints stopping most people. And it's not like you can exactly fail a language. If you give up, you might stagnate at whatever level you've reached. Maybe you'll even forget some things. But you always have the opportunity to learn more and practice. Recall practice doesn't even require any equipment or opportunity other than a quiet moment: Just form some sentences in your mind. See if you can express an idea. Maybe check and look up some hints afterwards. I find that's a good way to start a snowball rolling and recall is the most important tool for pushing the brain.