Because eggs were 3rd on the list of why I considered getting chickens, it never occurred to me that there was a schedule they should keep. I mean, I read a lot, and it looked like somewhere between 18 and 24 weeks, they might start laying unless they were slower maturing breeds. I had a couple of those in my very first batch o' 8 chicks. I marked up a calendar, because I originally had 4, then added a couple, then a couple more, so I had different ages of chicks. I needed to know when they ALL reached 8 weeks of age, to take them off the medicated starter feed and consider putting them outside full time. (The youngest was 8 wks old and the oldest was 12 weeks old; I wanted to put them all out at one time to minimize multiple pecking order periods.)
Then I just wrote more stuff down after that, plotting out ages. I did put "look for eggs" on week 22 for the oldest chicken. Then I added two POL hens to the flock when someone on BYC was down-sizing HER flock. Totally forgot about that calendar as time passed, and when the first egg from one of original "chicks" appeared, I noticed it was "early" when I happened to look at that calendar. Two more started 'on time' and the other four (one of the 8 was an accidental roo) laid their eggs "late."
I've never had kids, but I've seen friends go through similar "milestone" issues with theirs. Like the second child, there wasn't any calender marking about eggs for the chicks I added this spring - who knew WHEN they might lay eggs; I didn't try to schedule it for them, not even as haphazardly as for the first batch.
There have been some amazingly impatient posts about waiting for first eggs. I'd rather be doot-de-dooting along and FIND one, for a pleasant surprise, than get all wracked up about my chickens being slower than others.