N.L.
I went through the exact same thing w/ my(now) 17 month old daughter at that same age. I had a little rule in my own head that I was not going to get her out of bed before 6:00, so I didn't. Not that it worked any magic but I refused to get her out of her crib at 5:30 a.m.! I do think it's important to not get in the habit of engaging them so early or they will keep doing it. My daughter is still very random with her wake up times. One week it's 6:00, one week it's 7:00. I can't seem to put a finger on what causes the differences. I'm very consistant with her bedtime so I know it's not that. Honestly, I think you just have to have confidence that it is nothing that you're doing or not doing to create the early waking. Unfortunately, I think it's fairly common at these ages. Their little bodies & minds are going through so many changes, not to mention the teething! Good luck, I hope it helped to at least know that you are not alone. :-)