[Missed Miss]

I would think 23 or 24 would be a better time to start. Your body's not completely done growing until your mid 20's. It IS possible that if you do stuff to the girls now, what growth you might get in a few years won't happen. I've even heard that they'd actually get smaller.

And no offense, but this is a bit overkill.

Agreed I have never heard women still develop breasts after 18 happening except with pregrancy some birth controls, weight gain all things that effect hormones or fat. Unless she only just started her periods she is probably good to start.
As for her BC and cycling it doesn't seem important but if she wants to keep bleeding regular maybe experiment with all month, stopping when she stops the pill or the half and half cycle or ramping up and down to find what works best.
Yes I'd say for me keep it was so much easier to keep it simple I only used pm and msm with my progesterone BC no cycle half ainterol pm dose everyday you should be able to use PC cream but just a little not everyday.

Third. I might hesitate to quite declare that ALL women are done with puberty by 21, but the ones who aren't would have some mighty strange extenuating circumstances.

21 is a great time to start.

Okay. On your own head be it. I'm just telling you what I've heard about a read. About 20 years ago, I was trying to put together a benefit album for breast cancer research so, I did a LOT of reading and research on the subject!