Paige, starting before age 25 still has some advantages even though you're already past peak pubertal growth.
There's evidence to show that the male skull continues to grow and mature up to age 25- depending on the individual, further frontal bossing and forward growth of the lower jaw growth can continue up to age 25.
So even if you do have some male facial skeletal features at age 21, starting earlier may have the advantage of stopping/reducing further growth of those male features.
As an aside, physiologically, in both males and females, peak bone mass is reached somewhere between ages 25-30 - there are some who advocate going as much weight bearing exercise as possible for this age group for this reason. Reason being - the more bone mass you have to start off with, the more you have to lose before you get to the stage where you may be considered osteoperotic.