“A few years ago one of my children took on a chumrah that I didn’t keep. My first reaction was negative — are my standards not good enough for you? Later, when I thought about it, I realized that I was dead wrong. My child wanted to improve herself, why should I stand in her way? I apologized for my reaction and encouraged her to follow her conviction.”
I once heard (probably somewhere here in the CR) of a woman who was complaining to her Rav about her son being too frum. The rav told her that all teenagers are rebellious in some way and this is his way of being rebellious and she should be glad he is not going to the other extreme.
Whether you want to call it improving themselves or being rebellious, the point is that you should think things through before trying to argue with teenagers.