My wife and I rode in 4 century rides this year, and we thoroughly enjoyed them all. I'm 58 and my wife 57. We did two on our half bikes and two on our tandem. I'd strongly encourage you to make it a goal for yourself. You'll find all levels of riders out there, so don't worry about how you'll fit in. Take it easy at first and don't get caught up with the boy (girl) racers, pace yourself and you'll do just fine.
There is an excellent book, "Long Distance Cycling" by Burke and Pavelka. It has wonderful information and a training schedule for working up to a century (and more). My plan is to attempt a double century next summer, but it may be a bit too ambitious right now. We'll see... I would strongly encourage you to obtain this book if you really want to do a century. It's packed with great advice.
For your first such ride you may choose a shorter ride, just to get the experience. Most organizations offer 30 or 62 mile options (a metric century--100 km). However, if you are riding 60-70 mile training rides, you should do okay with the 100 miler. It will also depend a lot on terrain and wind. If you want to gain experience riding in groups, you may want to find a riding club and join them in organized rides. Our club, Bonneville Bicycle Touring actually sponsors two century rides, the first (and only one of two in the USA) woman's only century ride (Little Red Riding Hood) and the ULCER (Utah Lake Century Epic Ride). Both are great rides with great support and lots of options.
Most century rides are supported and you won't have to worry about food or drink, but the level of support can vary. I take a small container of powered sport drink (Gatoraide) to supplement my water, and a few gels for a lift during those dark miles. Normally the sponsors will provide details about their level of support. You'll feel a sense of accomplishment and do something no one can take away from you. Go for it! OHB