Here are the sentences rewritten using double comparatives:

1. The older she gets, the more thoughtful she becomes.
2. The more frequently she practices, the stronger athlete she will become.
3. The more money they save, the nicer vacations they can afford.
4. The more frequently I exercise, the healthier I feel.
5. The more fluently he speaks, the more confidence he will gain.
6. The longer hours they work, the higher salaries they earn.
7. The more diligently she studies, the better results she will achieve.
8. The more regularly he practices, the more he can improve his skills.
9. The faster they run, the more races they will win.
10. The more books I read, the more my knowledge expands.
11. The more efficiently she studies, the better she will comprehend complex concepts.
12. The harder he exercises, the more muscle mass he will gain.
13. The more effectively they collaborate, the higher quality work they produce.
14. The more strategically he manages his time, the greater productivity he will achieve.
15. The more wisely he invests, the greater financial returns he will earn.