They say that timing is everything. While it may vary in prominence and importance for a story, it’s always a good idea for a writer to know how long it takes for things to happen. Having the ages of characters and timeline of events written down and referenced periodically during the rewriting process will help you maintain both continuity and pacing.
Note that I said, “during the rewriting process.” Timelines and continuity checks are part of the many cycles of editing. Unless you are one of those ultra-detailed planners who lays all of the groundwork before picking up a pen, a timeline isn’t something you should be using until after at least the first draft is complete.
For example, when I write, I usually have a month or so of planning where I pull together a basic plot line, character descriptions, and overall tone of the work. In the character descriptions, I put at least an estimate of how old they are supposed to be. This can fluctuate later, but usually only within a few years of the initial age-setting. As I write my first draft, I have a rough idea of how much time passes between events. It’s a day or two from their initial meeting to their first fight, a week until their marriage, a few hours until that important breakfast, and they spend two or three months in this locale. These aren’t set in stone, nor do they have to be 100% accurate at this stage. In Draft 1, it doesn’t matter so much if I say it only took a week to travel 200 miles on foot or something like that. All I need are estimates, if that, to give a basic temporal framework.