I don't think one would normally worm birds before this age, unless there was reason to think that they came from a dirty place where they had been poorly kept and exposed to heavy risk of infection. (And unless they were 'rescue' birds you wouldnt be getting them from somewhere like that anyway!) When I get new pullets I normally worm them when my older birds are due for dosing, then after that they are all in synch. for doing at the same time. If I didnt have any older ones I would probably do the POLs some time between 18-24 weeks so they were treated before they came into lay. I've found that it takes them a week or two to settle down enough to eat normally, so doing it as soon as they arrive might not get the full dose into them.
When I've raised birds from chicks or young growers I've always given them growers food, for the reasons you say, Margaid, also the protein levels are less in growers food so they grow on more slowly and steadily. You have to give them something, so it might as well be the right stuff! Ideally, yes, it would be good to continue this until they come into lay, and with 18-week-old purebred POLs at this time of year this may be 10 weeks or more ahead, not until after Christmas. But it's also a matter of convenience and avoiding waste - you don't want to be left with most of a big bag of growers feed if you've only used a small part of it between 18 weeks and start of lay, for just a small number of birds. Over 18 weeks it probably doesn't make a lot of difference what sort you feed them, so would be OK to use layers feed if that was best for you.
