My DS has been working on them on and off since around 26 months and none of them are through the gums yet. His main teething sign is drooling. He was always a big drooler, but stopped around the time he turned 2. In the last few weeks, he has started drooling again and I took a quick poke back there and sure enough, they feel very close to breaking through.
Lindsay - Mum to Logan (May 3, 2007), Graysen (June 26, 2009) and welcoming somebody new in September 2011!