If the screwdriver is 7.2 inches long, then the snake is just over 31 inches long (based on a computerized "snake measurer" program). The average size for a hatchling ball python would be closer to 16 - 18 inches (roughly). Growth rates vary greatly among ball pythons, and a lot depends on feeding. It appears that your snake has been underfed for a considerable length of time, so it would be pretty much impossible to come up with an accurate age based on length. However, I would GUESS at least a year old. If you wish to feed mice, I would suggest 1 mouse a week for a couple weeks, then give 2 mice (one at a time) a week for a couple weeks, then maybe bump up to 3. A good rule of thumb is to feed 10% of the snake's body weight. A better option might be to feed her small rats (still once per week). As thin as she is though, and since you are already feeding mice, I'd still go with 1 mouse a week for a couple weeks, then 2, then go with the small rats. Give her a chance to build up her gut flora before feeding too heavily (if she hasn't eaten in a long time, a large meal could be a very bad thing).
Best of luck!
Steve