I think the ability to learn a language is often related to how much uncertainty we can currently handle. As I get older, I feel myself and others pressuring me more to "know the answer" to things and don't believe I have as much room to be wrong as I did when I was younger. When I was learning Swahili in Tanzania in my 20s, I was wrong a lot but I guess I wasn't as worried about the consequences.

In short, learning a new language can make us look stupid and as we get older, we may feel more afraid of looking stupid :-D