15 is deffinately not too late. When i did taekwondo 5 years ago there was a guy there on black tip who was late 40's at least so just think that you're about 30 years ahead of him already. I'm also considering taking up something once i finish school. I'll have a load of spare time.
It is definatley not too late to start, I agree that you should stick with one art at first before mixing them. You might be able to find a good place that has a class that teaches an integration of arts though, like an MMA class.
i dont think its too late at all. i know people that have started in their thirties even, i think as long as u have perseverence and dedication you can definately do it.
It's never to late to start but you'll have to be patient cause it takes years no matter what discipline to become an expert and try not to be too disappointed if 12 years old beat you up at training because they have started at 8 or 9 years or even before that. That's the only thing that could be really unmotivating.