The amount of estrogen you take only matters up to a point. A body can only absorb so much. A small dose will likely cause slower growth than an average dose. Once the hormone receptors are active they are active. They do not become hyper active.
Age seems to have little if anything to do with breast development from what I have seen, although it used to be what people said and old info dies hard. Age does give testosterone more time to do its work, and much of what T does is unreversable with HRT alone.
There is some debate over placebo. The first effects people tend to report have to do with feeling calm and less anxious, this may be a placebo or it may be the catharsis of starting HRT finally. That said, estrogen hits the brain with surprising speed, depending on how it is administered.
Breast development, and nipple development are frequently the first noticeable physical changes caused by estrogen and it is not uncommon for things to start within a week or month. This is not placebo. But it can take much longer for some.