Gaining a solid understanding and fluent use of a language other than the one or two we have grown up using can be more difficult than when we are young children. There are are two factors that can contribute to making learning a new language more challenging:

1) In the first three years of life, when our first language or even two languages are being learned brain connections are developing at a high speed, and that makes learning easier.

2) During the time when people are learning their first, or first two, language(s) they are immersed in the first language(s), which results in their brain's gaining an extraordinary number of opportunities to build connections. There are many facets to learning any one,individual word in a language (recognizing it visually, recognizing it when it is heard, knowing how to spell it, knowing everything associated with what it means, pronouncing it, etc.) The first exposure to each new word has been described as being similar to an out-of-focus photograph, which, as the word is heard/seen repeatedly over time, gradually comes more and more into focus.

Living in the world, hearing words used, and being at a stage in brain development that facilitates the learning process makes learning first languages come naturally.

When people who are past their first few years of life try to learn a new language it is not possible to recreate the combination of degree of immersion with the stage of brain development that made learning the first language(s) so effortless. Many believe that the most effective way of learning a new language is through immersion, so being a pre-schooler is not required for a learning language through immersion. Still, the process of forming brain connections associated with a new language is not as natural and high-speed a process as it is when a child is in the earliest years of childhood.

This is, I'm sure, a casual and non-scientifically accurate version of why learning additional languages later in life is more difficult, but I compare it to a compare with a computer and its operating system, which must be present before the computer can do anything else. There's on one operating system, and everything else that gets installed later is a program. Our first language(s) could be seen as our operating system, and any new language we try to learn later are more like programs we install within that operating system. There's just a difference between an operating system and a program.