Why It's Harder for Adults to Learn a Second Language
Neuroplasticity is the culprit for the difficulty or ease of learning a second language. Basically, the brain is more plastic and flexible when we are younger. The brain is better able to make neuro-connections, which makes learning easier. Very young children's brains are basically programmed to be sponges; they soak up all the information around them. Every parent with a child has experienced the mortifying moment when their toddler or preschooler repeats a naughty word in public. Children learn their language through experience; a second language is no different.
But the brain begins to lose its neuroplasticity as we age. We certainly can learn new things, but perhaps not at the speed or with as much ease as we used to. Because adults have a larger vocabulary than a child, learning new words can easily be mapped over your pre-existing knowledge. An adult can learn to associate chair with la chaise, or library with la bibliothèque. However, learning the grammar rules or even the pronunciation is much harder, since the sounds of a language are naturally picked up by children.
You Can Still Learn Another LanguageDon't be overwhelmed by the fact that children seem to have a better grasp on bilingualism than you. Little Billy might have all the pronunciation of a native Argentinian, but most children speak in simple sentences with basic vocabulary- just like they do in their native language! Age is only one factor in fluency; motivation, personality, and the learning environment are also huge factors in learning a new language.
And some adults do in fact achieve near-native fluency. If your goal is to become bilingual, don't let yourself be dissuaded. You can learn that second language!
If you already know some of a foreign language but want to get better, you can join any number of communication groups through Meetup. Immersing yourself in another language is the best way to pick it up. You could get an app, like Duolingo or Mindsnacks to practice with. If you have children of your own, you could enroll them in classes, and have them help you with your grammar and pronunciation. Cultured Kids Club does offer Adult Spanish through local schools, as well as Parent and Tot classes in French, Spanish, and Chinese, which would allow you and your toddler to learn together. Whatever you decide to do, as long as you commit to it, you will be successful.