I did it over 3 months 18 months ago at 51, so you're not too old. I have heard the one month course is really intense, and you have no time outside of it, which would not have suited me.
If you're an experienced teacher, you will be fine with that side of it. For the grammar side, the 3 we had who were non-native speakers did better in some ways, because they had had to learn English themselves. I have done a lot of foreign language learning, so was in a better position than the other native English trainees for geammar understanding, but CELTA does use some different terms than I was used to. (I liked Teaching Tenses by Rosemary Aitken because of this.) Being a current Welsh learner was very helpful.
I did internally raise an eyebrow at one of my fellow trainees not understanding what was wrong with "he could of done that," but went into an explanation about auxiliary verbs instead. You know the grammar when you use it, you might not know the metalanguage around it and why we use certain word constructions as we do, so if you have at least a basic understanding of the parts of speech before you start, e.g. what an adjective or preposition or conjunction is and what it does, that will help you.
Lots of the books you could use are available online at much cheaper prices than brand new, though quite probably the school you're at will have a library of reference books.
Depending on where you are (university and seaside towns are probably better than where i live,) there might not be many jobs around, and those there are will not be well paid. Round here, it's mostly teaching for asylum seekers rather than language schools. Mind you, the way the USA currently is might mean people want to come here rather than the USA to learn.
Anyway, for financial reasons, I'm back in IT rather than teaching English, but I think about offering 1:1 classes/tutoring every now and then.