The answers above are spot on. However, I thought I'd share with you my personal experience. English is my first language, but I grew up in Brasil, so Portuguese was my second language growing up. As a young adult I had the opportunity to work in Mexico so I taught myself Spanish. Since their are so many similarities between Spanish and Portuguese, it was easy to pick up. As a result, I have been able to claim a proficient fluency level for all three languages. That led to a fantastic position as a member of a Latin American headquarters team for a major corporation- that started my career.
My personal experience is that if you grow up learning at least two languages, you understand the structure and rules on how different languages play with each other- so it is easier for "us" to pick up additional languages. Take for example my siblings and I:
As stated above, I speak English, Spanish, and Portuguese Fluently.
My sister: English and Portuguese. Then she taught herself Spanish to such a proficiency level that she is now an American high school advanced placement Spanish teacher.
My other sister: Again English and Portuguese, then she taught herself French, which led to several lucrative corporate overseas assignments in France..
My Brother: Again English and Portuguese. But he also didn't stop there! He studied French in France at university, and then taught himself Japanese, Russian, German, and several others. He now speaks a total of 12 languages! Seven of those fluently! He is living abroad (12 years in Tokyo, and several in France, and some years in Russia). He has been working successfully for a translation/interpreting company as a freelance contractor.
The skill to speak multiple languages certainly has opened up a variety of opportunities for us.
Hope this helps!