POL means point of lay - from approx 16 weeks until they lay. There is not an exact age for when they lay as this depends on the breed, pure breeds tend to come into lay later then hybrids, also depends what time of year they were hatched too.
Cant help with an organic alternative to amprolium, but in my experience natural remedies are only a preventative not a cure. I use garlic & ACV as a preventative but still worm the chickens with flubenvet twice yearly. Wild birds carry all sorts of nasties.
You could say have worm counts done on all your animals then worm as/when needed chemically.