What are the prerquisites for attending yeshiva for the first time as a middle aged man without a strong background in Yiddishkeit? The yeshiva graduates I've meet seem very intelligent, does one need to be at that level to study at a yeshiva, particularly as concerns yeshivot geared towards baalei teshuva?
There are many yeshivot geared specifically to both Israeli and foreign balei teshuva in Jerusalem. Admissions criteria vary, but in general their mission is to educate Jews and they are not selective. It would certainly help to have a command of the aleph-bet (Hebrew alphabet) and some knowledge of Chumash (the five books of Moses) before entering a yeshiva, but the only hard and fast rule is that you must be willing to keep Shabbat. Most of the time you will be living in a dorm and/or attending meals and shiurim on Shabbat, and I don't think a single yeshiva would tolerate on-site Melacha Shabbat. Kashrut is not so much of a problem- all yeshivot provide meals, and you can be reasonably confident they're kosher- but you would do best not to be seen eating a dairy ice cream after a meaty lunch!
In terms of raw intellect, the Torah meets everyone at their level. I have seen very learned people in secular subjects struggle with Torah learning, I have seen people who seem rather simple grasp Torah concepts with ease, and everything in between, and, most remarkably, I have seen people who were clearly not born with the highest possible level of innate intellectual ability who have become truly brilliant through years of Torah study. The key is bending your own mind to the Torah rather than trying to bend the Torah to your mind. I.e. accepting, for example, that not keeping chametz in your home over Pesach is not "stupid" or "outdated" but rather that G-d and our sages know and understand something that you don't. This is not an admissions prerequisite but will certainly bring you much success.
In the intro of Jose Faur's dissertation on the "Guide for the Perplexed" (thanks Alex) it says: A dimension of Hebrew perfection is the desire to share it with others.
In that regard, there will be a place for you. I don't know what stage of life you're in, but as a bal ha'bais, I would go to a local yeshiva several times a week. Fortunately a brilliant bucher was willing to teach me.
On a practical note, I would echo what Josh K said above. You might consider taking some steps on your own as preparation and also to see how much intellectual effort you're willing to commit.
Not that you should necessarily do as I did, but you might begin by teaching yourself lashon kodesh (the Hebrew of the Chumash). There are several such books available, many more so than what I used 40 years ago. Then you can try to translate some of the early parshahs - quite an uplifting experience. Also I used Brown, Driver and Briggs Lexicon which can give you a breadth of the meanings( https://hebrewcollege.edu/wp-content/uploads/2018/10/BDB.pdf) and a grammar (I used Gesenius - http://tmcdaniel.palmerseminary.edu/GeseniusGrammar.pdf).
You can also access a number of daily mishna shiurim on-line.
These are foundational yet exalted. However, in my personal experience, once in the yeshiva curriculum, the focus was on gemara. As you may know that is largely in Aramaic. I preferred to learn l'iyun - which is a glacial pace (along with the commentaries of Rashi and Tosfos found in the side margins of each page (daf).
Studying at that pace, you can do a small section at a time, and really get the flavor (tam Gan Eden). No need to get anywhere - ayn l'dovar sof (no end to the matter) and "swimming in the sea of Talmud" are applicable.
Only attend one that has classes in basic Torah Judaism: introductory halacha, Hebrew, introduction to the mitzvos, a forum for questions, Mishnah. Some places just toss you into a Gemara shiur and don't even give an introduction to that. Consider staying in America. Toras Dovid in Monsey is a possibility as are the Chabad institutions in NJ, Miami, and Brooklyn. There's also Shar Yashuv in Far Rockaway, NY. Going to Israel is too much for some people. The environment there is very intense, and you'll hear all kinds of propaganda of how you should never leave the yeshiva or Israel. You might also consider just going at night or attending classes online. There are zillions of on-line classes. Disruption of one's life to go full-time is not productive for some people and once you move into the yeshiva building, it's not so easy to leave even if you don't like what you hear there.