I think its important the narrative here.
if u write a story and u say, she's a little girl of 14 years old.
that of course will imply underage
but if u write, shes a 18 years old babe that looks young, its ok.
in irl, theres girls that are 14 that looks like 18 and 18 that looks like 14.
so it can be confusing. I say she looks young, I could confuse her as 16 years old
but she could be a really young looking 18 years old as well.
edit:
and no. u dont need to make her boobs bigger. theres old girls with small tits and its normal.
I think she's ok as long the narrative says shes at least 18.