From what all I have read on here(and it's been a BUNCH!) You're going to get mixed answers on this. Some are saying you shouldn't start on Layer until you get that first egg. Others have said between 16-18 weeks.
As for the growing pellets...I searched and searched for this. My gals were just too big and didn't like that starter/grower "crumbles" which was almost powder. I went to 5 different feed stores SEVERAL times,asking for these,and every one said,nope,don't carry grower pellets. Well,one day one of them got some in. Said they were just going to try it out and if it sold well,they consider keeping it in stock. So I bought a bag. The pellets were about half the size of layer pellets and about half as wide. My girls LOVED them. So,next week went in to buy another bag and they were sold out. Man told me they went over so well,he's going to be getting some in and keeping it in stock. So,I'm having to wait till more comes in to get more. I ended up having to buy the darn powder,umm,I mean crumbles this time,but no more hopefully! So,just keep asking for it...maybe you'll get lucky like I did and a place will order some for a trial run.