I'm not sure, however, where people are getting this idea of what you can and can't do now that you're 40. I think it's pretty widely accepted now that you don't have to go for the helmet cut and sport cullotes. What I DO think is that people can pretty quickly spot the "desperate" 40 year old -- who dresses like they're 20 b/c they're desperate to BE 20, desperately seeks validation from the opposite sex b/c they're terrified their looks are going, parties perpetually like they're at a frat function, etc. I've especially seen the latter a few times and it is painful how they need to prove what great fun they're having (and usually acting like obnoxious jerks in the process). Seriously, if a person's life and relationships at 40 are in as much chaos as a confused 20 year old, then why wouldn't they want to grow up?
On the flip side, I don't think people give much of a glance at people who seem comfortable in their own skin. Whether you're dressing in so-called "young" fashion or just comfort, going out or staying in, if it's what you are happy with and you could give a rats about anyone else's opinion...well...to me, that's where 40+ kicks butt and people respect that. But if you run around trying hard to hide your real age, then it tends to just highlight your real age. In all things, grace, dignity and self-respect are ageless
Just last summer when my insanely hip 16 year old niece was visiting, I made some joke about remembering fondly the days when I wasn't double the size she is now. And she said to me, "Maybe you aren't, but you know what...you OWN it. You always look good." I am overweight, don't dress in top fashion (let alone the same shops as her), I wear comfy clothes and shoes, and half the time my hair is pulled up. So seriously, if a 16 year old can say that to me with a straight face, then I must be putting the right vibe about there. So there you go...do what you want. But OWN it