Are you kidding? Celebrities over 40 aside, there are plenty of 40-somethings that can get away with tight jeans with no problem.
My sister-in-law and best shopping buddy is 43 and is a total hottie. She dresses my age (29) and carries it off better than I do. She wears skinnies, leggings, and babydolls, and looks totally adorable doing so. If I look half as hot as her when I am that age, I'll be wearing tight jeans too.
Of course, you need the body whether you are 20 or 40, and everything has its place. Probably not appropriate to wear skin tight skinnies when you are 40 with three young kids going to soccer games and PTA meetings.