Becoming a concert pianist as a late starter requires the same things as an early starter... talent, luck, skill, quality of teaching you receive, luck, dedication, connections -- did I mention luck?
Seriously, I began my formal training when I was about that age, and am currently finishing my degree in piano performance. I'm on a piano scholarship and have consistently placed high or won many competitions in the past few years. This is not to brag, but just to let you know that it can happen. Starting late is not an automatic "oh, you started late, so you're doomed" type of thing. Until you're about the age of 12 or so, you can't really understand the music, your muscles aren't as responsive and although your brain can learn concepts, they really don't mean much until you're that age.
At age 16, if you're considering pursuing a career in piano, you should be doing all you can do to prepare for college auditions and such. For instance... maybe it's time you stopped with the grade thing and got into some actual music? There's so much more you can learn from that than the slow approach used in many graded methods. Those are good for awhile when you're starting out and learning basic techniques, but you need to start building your repertoire soon.