> Those who knew three languages, however, were diagnosed 6.4 years later than monolinguals, while for those fluent in four or more languages, enjoyed an extra nine years of healthy cognition...

Glad to read this...

I speak ~4, english being the 3rd (far from native but good enough to do biz) and the forth being basic/intermediate level. I've met people fluent in 5 languages... but they start to struggle with the 6th onwards.

That being said, I think 30+ is a lot. I've never known anyone who speaks fluently more than 6, but hey, if they can... great!

This people must have lived in a lot of countries since their early years, or their parents were polyglots themselves. I think the key is early exposure to different languages. You can easily teach kids 5 languages before they're 12...