At what age are babies able to perform sign language?
Most babies won't be able to demonstrate the sign back to you or make it on their own until sometime near their first birthday. Some may do it sooner, some later. This is due to the age of maturation of their hand muscles and in the language processing sections of their brains.
Research has shown the ease with which they pick it up corresponds to how soon they were exposed to it and how often they saw it. For example, a 13 month old baby who saw signing demonstrated several times a day for six months would have a much easier time of signing by him/herself and would most likely learn new signs at a faster rate than a baby of equal age and intelligence who only saw signing in the previous two or three months.