Hey callpepe. So far most contributors have just told you their favourite discipline, without any advice specific to your situation. I started tae kwon do at 20, practised for about 3 years, and resumed again in middle age. I also did yoga then and now. In between I mainly ran and did weight training.
First recommendation is, be nice to your body. Younger guys can punish their bodies and they get stronger, if we punish ours we get injured. Keep at home hot packs, cold packs, wrist ankle and knee supports, a heat lamp, liniment and anti inflammatories.
Second, I really recommend you get a yoga book or dvd for beginners, and do some consistent flexibility training , for about a month, before starting any MA training. Alternatively any good book or technique of stretching for flexibility.
Third unless you are already doing some sport I would also try to fit in some general toning up before you start, such as running. Get massages, training shoes, and start changing to a healthy diet.
Fourth I would consider the soft style martial arts first. The names are Ba Gua, Hsin Yi and Tai Chi or Chi gong (qigong). If you can't find these near you, read the descriptions of the schools carefully and ask whether they are derived from hard or soft styles. Aikido is partly hard and partly soft style, but has as a constant concern that no one is injured, even an aggressor, so it sounds safe. One branch of aikido also focuses strongly on the cultivation of ki energy; hard styles tend to let it happen without thinking about it, whereas soft styles address it explicitly.
Fourth, above all, talk to the instructors of the class you are considering joining. Try to talk to the head of the school. Make sure they are supportive of an older beginner. Make sure they understand your limits of flexibility and history. If possible, find a class which is specifically for adults and maturer people. Ask whether any instructors can give extra lessons. Always attend a class or two as an observer first and make sure it is what you are looking for. You have to be comfortable with the style of the training and the art. Make sure they don't behave like street thugs in the dojo, and that the instructors supervise properly. Make sure they use protective pads when needed (in free sparring), and that unsafe contact is not tolerated. There are lots of clips in Youtube which will help you sort out the differences in styles.
Make sure they are gay friendly if you can. When I joined my last club I was amazed to find that two of the seniors were lesbians who had just had a baby, just like QAF, and the occasion was joyfully written up in the club newsletter.