I'm kind of contemplating the same thing - not leaving the corporate world, because I have too many bills and debts for that - but getting a PhD in something, maybe math or CS. I don't know that anyone really does that in their forties, though...
This is on my mind too.
Am an engineer (EE + CS) with 25 years of work experience, with a passion for Physics. Am widely known in my circles as a scientist/physicist, however, I do not actually know much. Learned some Lagrangian and Hamiltonian classical physics recently.
I personally do not mind going for even an undergrad in Physics if that would be a better fit for me to learn. :-)
Best of luck!