I never said adults stop learning. It would be terrible if we did.
But spoken language, absolutely true. For example, my wife speaks three languages fluently. But she dreams in Bulgarian, because that's what she is. I've been programming since I was six. Now, almost 50, I see virtually everything as a process (which is not a good thing, just as my wife). I strongly suspect that I think as I do because of what I've been doing every day for over forty years.
I didn't mean to offend. Clearly adults can learn and many can reach professional level with strong dedication. You are obviously one of them. But I still suspect that the children the OP teaches will get far more out of the lessons than the parents if they are taught in the same manner. Ask yourself if you ever think, count, dream etc in Arabic?