Staring is not an involuntary behavior. You may want to think about what is causing this situation (with younger women) Is it due to boredom in the relationship, feeling as though you're having a mid life crisis, not being satisfied intimately/sexually, etc.
Behavior is generally purposeful and it is possible that you are doing the staring if if on unconscious level. It would be a good idea to do some thinking and see if any of those things ring true or if there is something else that may be triggering your reaction.
It also helps to remind yourself that looking is ok; staring is rude and that it may be the staring that your wife is having a difficult time with. Men are more visual than women, so you would naturally do what you're doing. Of course, if it is bothering your wife to the extent that it is, it may be a good idea to talk about it with her (when she's not upset) and also to try to self regulate your behavior. As long as you're aware of it, then you would have to keep an internal dialog to let yourself know looking is ok staring is not.