Hi I am 62 may and have been going to the gym now for a year. My arms are thin and had lots of stringy lines, mainly towards the inside ruining down to the elbow. I was very embarrassed by them. I have been using light weights,dumbell weights, at the gym and home. The other day I looked at my arms, and couldnt believe my eyes. They have improoved by 80% and look so much better.
Thought this might help you, for if I can do at 61 you can at 55. I also power walk and jog on the treadmill 3 times a week as well.
It is completely possible to still gain tone at your age and unfortunately it is a common misconception that you cannot. The only thing that you really need to remember is that you need to start off slowly when you begin any exercise.
One extremely good exercise for toning would be yogalates. It combines yoga and pilates and it is suitable for most people no matter what their age. There are even people in their 70's who practice it. So at 55 you are young in comparison!
It is all about how fit you are personally. Perhaps starting to walk would be a good start. Walking does actually burn off quite a lot of calories and it shapes every part of the body. It will help you to become fitter whilst toning at the same time. Swimming is another really good exercise which should help.
There really is so much that you can do exercise wise to tone up flabby areas; it just depends what you like and what you want to do.