At 16 months of age, our twin sons have begun to demonstrate that they understand the older kids and adults around them. Better still, each of the boys are doing a great job of making us understand them back.
While they aren't talking to us much, they love to grab our finger and lead us to places. For example, they'll bring you to the backdoor to indicate they want to go out and play. They bring us to the garage door to suggest they are ready for a car ride. They lead us to the "sippy cup cabinet" and point as way of asking for drinks and to the "snack cabinet" to indicate hunger.
Declan, one of the twins, is quite good at repeating a lot of words. He says ball, mama, dada, JC, baba, and a few others. Both twins can go get objects we ask them to get too. Upon request, they will bring us a football, truck, or even their binky or drink.
One of Cormac's only words is "Lulu" our dog's name. He says it perfect everytime he sees any dog. A dogs (in person and on TV is a "Lulu" to Cormac) and he simply adores the true "Lulu" following her around constantly.
The twins have also developed a non verbal communication system with each other. It seems to require little more than a glance at times. One looks at the other and off they both go to cause mischief typically. A couple giggles usually give their trouble away if nothing else does.