We used to think that this is true. In fact, we used to believe that it was almost impossible for an adult to learn a new language fluently. We now know that that isn't true. There are many plurilingual people (people who know more than 2 languages) who continue to learn new languages every few years as adults.
It can be a bit harder, and there is some evidence that our brains have less plasticity as adults, but it is possible and definitely worth it.
In today's world there are also many more ways to learn than before, which means it's easier to find what works for you!