People don't biologically change on their 18th birthday. They do biologically change at puberty. The point is that we no longer in our society consider that biology is the most important consideration. To insist that it is - but only in the case of transwomen - doesn't make sense.
To make the glaringly obvious point: age is a continuum, but sex is binary. A man can't become a woman, and vice versa.
But people do change age. (I can't believe I have to explain this.) A 15-year old will one day be a 16-year old, who will one day be a 17-year old, and so on and so forth. Children reach puberty at different times. They reach emotional and intellectual maturity at different times. (Some of them never reach it, mentioning no names.) Therefore the law has to make somewhat arbitrary decisions about the age when a person can consent to sex, vote, drive a car, get married etc.
The law doesn't have to make decisions about what constitutes a male or a female, however, because these are fixed states that don't change.
This really isn't difficult to understand, unless you are the sort of person who is determined not to understand it.