Dear N.,
Yes, it probably is his age, and it is time for him to learn to respond and be respectful when he hears your voices. It is just a matter of making sure that if he does not respond the first time that a consequence will happen immediately. Nothing violent,but you WILL turn off the computer, the tv or whatever it is that he is paying attention to.
Now, remember, in school - paying attention is an important skill. So now we know that he can focus on a subject, he has to learn to obey.
If you simply make absolutely sure that he can physically hear you - like don't call from the kitchen to his room or living room, go to a decent distance to him and call. If he does not respond, then turn it off. No threatening, no advance notice, well, you can tell him what the new deal is going to be - that would be polite of you - but do not say over and over that you are going to turn it off, just do it. He will be very upset, so that is fine - but he must realize that you two are his bosses, and that he must respect you. Use that word. It will get into his brain if you are consistent and work together. Do not disagree about what to do with him in his presence. Make your Dad and Mom contract and stick to it.
Happiness will come, really. Sincerely, C. N.