Over time, I think that you may expect improvement via the following:
- "Forgetting" bad habits
- Adding to your experience in non-chess-specific but useful skills, such as reading the opponent, time management, and recognizing your own limitations.
- More capable or willing than before of purchasing help (lessons).
Specifically to your question, it's worth noting that 1 0 bullet is a far different game compared with 3 0, or 5 0. Also, sites will differ, and the site you mention (Yahoo) has changed over time. As you probably know, the specifics of each site's implementation has a dramatic effect on optimal strategy. For example, does the site allow premove? If so, is there a minimum amount of time which must come off the clock? So leaving 1 0 bullet four years ago and returning to 3 0 today is a far different game.
For the issue of whether you should be faster mentally at 25 or 30, I don't know if the difference would be discernible. But this is not my field of expertise.