Based on the information from this site children are usually able to answer simple questions (like the example you have given) by the time they reach 3. At 2.5 years most children are able to answer most yes/no questions. You can also find hints how to improve your child's answering skills.
Remember that each child is different. One may start doing something earlier than other, but start doing something else later.
I think there is no reason to worry yet. But if you are in doubt or already worried, do not hesitate to consult your pediatrician.