The only pullet I would personally put on layer feed at 15 weeks might, might be leghorn or ISA type commercial pullet that generally begin ay at 17-18 weeks. With other breeds, the POL, especially in winter, is often 24-28 weeks. In other words, your 15 week old chick, might be only half way to POL. High calcium for the slower developing breeds for 8 and 9 weeks before POL is proven to be detrimental to development. Research it for yourself. I've no dog in this fight. My pullets are healthy and I'd have no interest in harming them in any way, but that's me.
There's no magic in layer. It is merely a raiser or grower with added high calcium. It doesn't make them lay, speed production or increase production. It is feed for hens who are already laying to make up the calcium needed because of egg shell production. That calcium "make-up" can also be provided for through offering shells and some studies indicate that it might even be a superior way to provide calcium.
At POL the pullet's laying is sporadic and she'll lay a very small, pullet egg. There is plenty of time to bump the calcium when she actually begins to lay, and that point is simply not predictable with absolution precision.
So, the above is for what it is worth. Been doing this for 50 years, so take that for what it is worth as well. The science is available for anyone to read as well. Best regards.