Why not dedicate a CD to her instead of a song. I don’t know exactly what you are planning for the Birthday event, but I think a CD would be something she could take home and enjoy over and over again. You could junk all our suggestions and pick songs that remind you of your mother. They wouldn’t have to carry the idea of getting older, motherhood, etc… They could be just songs that are special between you and mom. For example, you might remember an album or tape your mom used to play when doing house work, relaxing, driving in the car.
Hope your mom has a great birthday!