4.5 months or 18 weeks is a good guide or average age to begin needing this requirement.
But bags are labeled for averages & for laying breeds, but many of us do not HAVE laying breeds, we might have dual purpose breeds for instance... with different needs.
The point of layer feed is that certain nutrients are decreased and others increased from the growth feed. You want to feed this mixture at the right time for your birds, every bird is different. The Right Time is when they cease their growth phase, much like a teenager stops growing taller at an "average age" and they begin the production phase of their life cycle.
If you start the production phase nutrients during the growing phase, you could stunt their growth
If you start too late, you could cause problems with their production of eggs...
In time you will get to know your chickens, specially if you keep only one or two breeds and you will learn what works best for yours.
For me so far, combs begin turn red 4-5 weeks before the onset of lay, that is my signal to start thinking about egg requirements, including nest boxes etc.
When in doubt, follow the manufacturers recommendations for sure! That is always the best option.