Some studies suggest that language acquisition gets more difficult as we age, although the evidence is not conclusive on that score. Learning a first language is almost always at an age where the brain and cognitive processes are still maturing and developing, which also explains why it is easier, and wiser, to teach children more than one language from a very early age. They are like sponges for language. The popular language learning program on CDs is based on the fundamental idea that language is best learned when it is not "scaffolded" to another language, that is, when we learn the second language just as we did the first.
I have seen similar studies talking about how children who are taught a second language early tend to have advantages in their cognitive learning. I have heard of some schools even beginning to over a second language class to Kindergarten students.
NPR's Morning Edition recently had a story on this subject you may wish to listen to: http://www.npr.org/2011/04/04/135043787/being-bilingual-may-boost-your-brain-power
The study they discuss found that children who grew up bilingual had some advantages in terms of cognitive development and cognitive health over monolingual individuals. It didn't so much discuss second language acquisition as an adult, however. Anecdotally, I know that one of the toughest things for me to keep straight when I was learning languages as an adult was keeping my foreign languages straight. I learned German in high school and college, then learned Hindi. I found that when I was stuck for a word in one, I would substitute with a word in the other language. After a while, and with some practice, this went away and I could keep these languages straight, as well as a few new languages I learned to read as an adult.
The other thing worth noting is that when you get to adulthood, it is extremely difficult, if not impossible, to achieve fluency in a second language. Being familiar with a few secondary languages made it easier to learn new languages, but only to learn to read them, not to speak or hear them.
There is a very big difference between these two subjects. Clearly, the way that children learn their "mother tongue" or native language is very different from adults learning another language later on in life. Children instinctively "learn" such aspects as grammar and complex constructions without the need to be specifically "taught" them, whereas adults need to learn such complicated structures. I have learned Spanish as an adult which has been a painful process, whereas my children have been able to grasp such grammatical features of Spanish as the subjunctive with an ease that has infuriated me!
Can you be more specific about what you want us to discuss?
At the most basic level, children learn their first language much more easily and without really needing to use their cognitive abilities. By contrast, adults learning a second language are really needing to think a lot more. They are, it is believed, using different mechanisms for learning than kids do when they are learning their first language.
Just to make a comparison (First Language Aquisition in Children "Vs" Second Language Aquisition in Adults)