It's important to realize that on top of genetics, sometimes inadequate HRT can lead to poor results in some, not everyone is treated the same and by the same doctors and some may have done better on much higher but still safe doses due to lower sensitivity.
Often, it is noted younger women will have better development but based on my 10-12 yrs of reading feedback from transwomen, this does not seem to hold true as there are as much disappointments as successes in both age groups, sometimes older women even have better results in terms of breast growth, for instance. It may also be the case that some hormones taken orally are metabolized differently in the older group with higher concentrations in the blood, as a result and thus stronger effects.