How can I be less childish?

This is an excellent question and an honest inquiry.  Personally, I see this as a journey in your own self-awareness and an opportunity for personal growth.  Your wife has made you aware of what she sees, but it seems apparent that you are not seeing it.  If she hasn't done so already, it is important that she be able to communicate with you when you are acting that way in her prescence so you can begin to explore why.  It is my assumption that you are potentially behaving defensively, which means that you could be perceiving that you are being attacked.  It is important to be able to ask ourselves: Is this true or is it only a reflection of how I feel?  What may be contributing to this thought or feeling?  What am I responsible for to regain my composure?  

As you explore this with your wife, it is important that you recognize that her feedback is not criticism.  When we feel criticized, defensiveness is inevitable.  Once you get enough information about this because she is helping you see in the moment what is happening, you will begin to hopefully be aware of what things may be more triggering for you.  Once you know the triggers, then you can begin to identify how to cope with them more effectively by creating some skills, like take a break, focus on your breath, count to ten, use a grounding technique.  

I might also recommend that you and your wife find a way to discuss this by using a gentle start up to communication.  The technique is:  

Start by stating your feelings. For example, first you (or they) can say, “I feel … [some emotion].” If you can’t identify a specific emotion, you (or they) say something like “I’m upset,” or “I’m stressed.”

Second, you (or they) can describe the situation that has evoked the emotion without blaming the partner for it. In other words, you (or they) describe what has caused their feelings, not who.

Third, they conclude by saying what you (or they) need or hope for to feel differently.

I hope this helps!