I have my own biases here, but I'm in a similar boat, a few years younger (41), also an NP (11 years total in nursing). I will be applying when I'm 43. I'm lucky in that I watched my Dad walk a similar path (he wasn't happy as a pharmacist, so went back at age 41 and is still practicing nearly 30 years later). So it can be done. In my case, it is true--the opportunity costs are huge--4 years without income and another 3-4 at about a third of what I make now. But I also work with terminally ill patients and they have taught me that all the retirement money in the world doesn't really matter if you have a ton of regret about how your life could have turned out and not following your heart (it sounds cheesy but could not be more true for me).
I am magnetically drawn to medicine--CALLED to it and can't think about doing anything else. I wish I'd been mature enough and had enough self-confidence to do this as a younger person, but I wasn't. That's that, I have accepted it and have moved on and am doing it now.
I think if you're ready and HAPPILY willing to accept the opportunity costs--retirement income lost, nights of sleep lost, loss of feeling like an
expert--that others have mentioned and you know you would regret at least not trying, what more do you have to lose?
Don't do it if you feel that worrying about retirement funds, having to deal with working months of night shifts, and taking direction from people with far fewer years of health care experience is going to grate your nerves so hard that you hate the process of becoming a physician, even if you'd still really enjoy the outcome. We're talking about the better part of a decade of your life, so you need to be sure you're going to enjoy the ride!