使用计算器插件/ jquery计算年龄

I have to sets of dates (via inputs/selects - mm/dd/yyy) on page. I want certain readonly text field to show age in months (calculated based on those two dates) after a certain select field (different select, not a part of dates set) has been changed (any option in that select chosen must trigger calculation and show result in readonly field). It should be pretty simple, but doesn't work, would appreciate any advise. This is what I have so far:I have to sets of dates (via inputs/selects - m