Happy B Day!
Coming from a guy who sees "39" from the rear view mirror I can say you've got nothing to worry about.  Even "40" is the new "30" or maybe it's "50" now.  - whose counting.  If I were you I would be more concerned about what your smart savy wife has planned for you every time you celebrate cause coming from someone like me who spends a lot of hours working in the same office every day with her I know she really adores you and will go out of her way to let you know it in very celebrative ways. SO enjoy your big day and make sure you thank the Lord for making some great decisions that brought Dawn and Leah into your life. You're a lucky guy. 
Rick