Tai Chi is a Wonderful art if it's with a good teacher. It's part of a "family" of three internal arts of China, with Hsing-I and Ba-gua. These art's did not all come from the same parent system, but there are enough similarities in principles that they are grouped together. Any of the three give additional benefits on top of having martial applications: health and longevity benefits are associated with all three. They are easy on the joints, easy on the body, and effective. Hsing-I is the combat military art of the three, designed to take territory. Ba-Gua is an old bodyguard art, with some of it done while walking in a circle- it's savvy and I consider it a sly man's art. Tai Chi is Tai Chi, it's sometimes hard to make work if you don't have a background in other art's applications, from personal experience anyway. If it's done right, it's a great workout.
At 64, you don't need to go through what is designed in a lot of hard arts to wear the ego down in order to learn something, or put up with the physical abuse that can occur in others as a right of passage. I could be more politically correct, but that's just kind of the gist of a summary.
Some of these forumites, like Danny, may know of a Bujinkan school in your area that would probably also work well. I'm friends with some of their early day veterans, and get along well with them. If you can get someone from the 70's or very early 80's era, you'll probably get a really good version of what their art, closer to how it was originally designed.
You always have to go by your instinct and guts about a school and an instructor. There's good and bad eggs in ALL the styles. Visit a few and go with what you feel right with.
One very bad sign, if you ask an instructor about other arts, and they put down other schools and styles. It's been my experience over around 30 years or so, that they will probably not know a whole lot, and you will probably get other fun issues out of them if you stay and train long enough.
Hope this helps.
David