It is very difficult and near impossible to get admitted to a medical program in the United States at the age of 45. Especially if you have not been in the medical care profession (such as being an R.N. or pharmacist) There is "age limit" imposed by the admission committees. There are alternatives, such as physician's assistant or nurse practicioner, that are doable.
I don't see any reason why you couldn't. There is no age limit and the only thing that you need to do is set your mind to it and give it your all. I wish you luck in your new endeavor.