I'm currently studying my seventh language, and so far I can only say that this is probably the easiest language I've ever learned. Perhaps because by now I can find similarities between this language and the ones I already know, or perhaps because I've figured out the most efficient way for me to learn new languages.
But all things considered, learning a new language literally means learning to speak all over again.
It's starting as if you were a baby, having to memorize which sound corresponds with which letter (if your new language has its own alphabet- and this is the 3rd one of mine that does), and then which words correspond with which objects or concepts.
It means your brain has to create some form of compartmentalization, which allows for translation between the languages while keeping the words from one language separate from the others. Otherwise, I would find myself unable to write a coherent sentence in English.
For me, this means I'm learning how to speak for the seventh time. I'm learning cultural references, sayings and common phrases. I'm learning which words are exceptional in their pronunciation or spelling. I'm learning which words have bad connotations, and I'm learning to intonate and to accentuate.
Like being a newborn again. For the seventh time.
We're never too old to learn anything, and our brain will never be full. At least mine isn't, yet.