Research shows that children are proficient at learning a second language up until the age of 18, roughly ten years later than earlier estimates. It also shows that it is best to start another language by age ten if you want to achieve the grammatical ability of a native speaker.
There are three possible reasons why the ability to learn a language decreases at 18.
There are many examples of people who pick up a language later in life. Our ability to learn new vocabulary appears to remain constant, but most of us will not be able to master grammar like a native speaker.
Research shows that people who learn via immersion are notably more fluent in a foreign language than those who are learning it in a class. If you have the choice between starting language lessons earlier or learning through immersion later, an immersion environment seems to be better.
Although it is scary and hard, immersive practice is by far the most effective. When the person you're speaking with sees that you don't understand, they will automatically try to simplify what they communicate.
If you don't want to speak yet, you can also try reading or watching movies, until you have a high listening comprehension.
It is a good idea to master the basic skills first and focus on the grammar later, while asking for feedback and correcting yourself, learning on-the-fly.
Invest not only your head but your heart in the learning process. Practice makes perfect.