Of course you can start a career that late, people do it all the time. My grandfather graduated college with his electrical engineering degree at 45. He's 83, and has only just in the last few years stopped working completely. And that's not because of financial issues regarding retirement or anything like that, he just enjoyed it that much. He retired long before that, like at 65, I think, but he still worked, with the same company after retirement as a "consultant." IOW, he was the guy in charge who made all the real decisions but he let some young kid fresh out of school have the title and feel important lol.
Having said that....going to work doesn't necessarily mean starting a career. You could go get some fluff job that is fun, but doesn't give you any sorts of real responsibility either. You could get some retail job that would give you a discount at your favorite store, get a part time ticket taker job at the children's museum so you get employee discounts for your (future) grandkids, that sort of thing. Jobs and careers aren't the same thing and there's no reason to invest time and effort into developing a career if it's your family, kids and future grandkids that really fulfills you. That doesn't mean get a job you hate, you can certainly still enjoy your job and still have it be a job.
I do have to address the "DH may well LET me" bit. It shouldn't be about him "letting" you stay home after the kids move out. It should be about the two of you discussing your goals for once the kids leave. He should be identifying why he wants you to work, you should be identifying why you want to stay home, and the two of you should be coming up with plans and goals for when that happens. Are you positive you are even going to want to stay home? If you have no kids to take care of, no house to clean (ie if there's just you and your DH and he's gone all day, who's going to be home to mess it up?) what ARE you going to do? That should be part of the discussion too.