Billvila hit the nail on the head.
I'd encourage anyone to learn anything they're interested in. The problem is that if you don't actually USE what you've learned, and regularily, so that neural pathways get established inside your brain, you forget it.
I spoke English to my father and he spoke Ukrainian to me. To this day I can still understand Ukrainian almost as well as English, but I can barely speak a word of the stuff. I recognize the words when I hear them, but when I try to speak in Ukrainian, I can't think of the word I want, and I can't pronounce it either. It took 20 years for those neural pathways on understanding the Ukrainian language to get built up in my brain, but the neural pathways to the memory to think of the words and to the muscles in my face and tongue to pronounce them properly were never used, and to this day I know what the word sounds like, but am unable to pronounce them. It's analagous to knowing what you want to draw, but when pencil comes to paper, it just doesn't look as good as the image in your mind that you wanted to draw.
So, keep that in mind. It's great to learn stuff, but unless you use it, the brain will tag it as "not needed" and it'll get replaced with other stuff your brain considers more important to keep.