I started learning Spanish at 51 (three years ago).  I spend a lot of time at it because I enjoy it.  I am not sure of my exact level but some of my teachers have told me I am advanced.  I had a new teacher today who told me that I have mastered the language (although that sounds like a bit too flattering to me because I still make plenty of grammatical errors and still often hesitate when speaking.)  

I read a lot.  I watch a lot of movies in my target language.  I am fortunate to have a service with unlimited private lessons so I also get more than 10 hours of speaking practice a week. 

What I have read is that adults are quite capable of learning languages.  I think that is true as long as your short-term memory is still intact (which it should be at 40.)