There's two schools of thought about progesterone. One is to take it only when in late (?) Tanner stage three and not earlier. Other is to start it right away. I started at was it seven months on HRT? Quite early, but I was obviously in T3 by then. Others wait for year or two but I don't think its necessary and it can be started right away. I have never seen evidence to the contrary, that early start would be detrimental in any way.
The fear of prolactin I do not understand. It is true that high (VERY high!) prolactin will inhibit both E and PG, but it being somehow again detrimental is a myth. Its essential piece of the breast puzzle obviously. How and when to boost it, is it needed, who benefits and so on is where the weirdness starts where I can't give any definiteve answers. All that I know for sure, is that I have hugely benefitted from boosting prolactin and I have a natural sensitivity to it.
This ties in with dealing with HGH. GABA is fantastic for growth hormone boost and it also boosts prolactin by a big margin. For me its likely the single most effective (and cheap!) supplement for both. GABA before sleep gives a big growth hormone surge during nighttime, exactly what we want to do, especially after 30 years of age, even more so as we get older. Before around 50-60 years, elevating growth hormone close to young adult or even teenager levels is not very difficult, as we get older same benefit requires more and more workout and or supplementation to make happen. It is absolutely essential, no growing happens without growth hormone, its also a precursor to a whole lot of stuff that we need for any growth and healing to happen. I think some supplementation of this is very good for health in general. Abuse obviously is not as that can cause massive problems, but we're not injecting it, just exercising and taking food supplements to support our natural balance. Another good one for growth hormone is L-tyrosine. There are bunch of amino acids that can aid with this, one such experiment I have in planning and I'm yet to get into it.
The thing about prolactin once more, is that it does a lot. It aids areola + nipple development, ducts and what ever else, it makes lactation happen and it can also help growing fat cells to breasts, guess what all very busty women have? Very high fat percetage in breasts. Partially its genetic, the potential for carrying fat in boobs, the rest is about what we do about it, how to help it. Elevated prolactin in absence of progesterone when its not making milk... Makes fat = big boobs. Obviously its not smart to go totally crazy with prolactin as that makes lactation happen and it can inhibit other hormones we want to keep high enough. I'm just scratching the surface here, pulling stuff out of memory and I'm not great with it. There's a whole lot about this to be found online with bit of searching, I just tend to think that importance of prolactin as a puzzle piece for breast growth is often overlooked and forgotten and people even are scared of it somehow.
Also, elevated prolactin levels shouldn't be something to be scared of. I have it quite high naturally, but much much lower than what a prolactinoma (Brain tumor which makes prolactin levels soar.) would do. From multiple lab results and my diary, I have concluded that several things easily elevate my levels and its been very beneficial for breast growth. But I'm just me and these things work differently to everyone. The thing is to not overlook these things.