When to change the feed

[Pele]

Just keep life simple for yourself and switch to layer feed when they start laying.

 

[Fred's Hens]

The confusion on what feed and when still amazes me given the number of times it discussed here. This site also has a wonderful Basics 101 type page for beginners.
Most commercial feed bags from Purina or TSC's Purina (DuMor) have the options right on the back of the bag.

Chicks and youngsters, before laying can eat anything. Flock Raiser, starter, starter/Grower, grower, etc. You pay your money and take your choice. It's all feed. Chickens have survived for centuries on catch-as-catch-can diets. Not saying that's ideal, just sayin'. Everything will eat a chicken and a chicken will eat everything.

Layer Feed, as the name suggests, is high in calcium for LAYING HENS. It's just that simple folks. If your pullets aren't laying yet, feed them anything else, they'll be fine.
If you have a mixed flock of hens and youngsters, and many here do, then simply feed Flock Raiser and put out shells for the hens to pick at as needed. The chicks and youngsters will generally just ignore it, as their bodies aren't crying out for the calcium.

BTW, most production/hatchery barred rocks will begin laying around 20-24 weeks, it just depends. They lay when they lay.