If that is what you meant to say, you really didn't explain that well in your original comment, as you explicitly claimed that the law could not take details into account, rather than that it should specifically avoid subjective criteria. After all, it is perfectly possible to take into account the fact that not all people of the same age have the same maturity without resorting to subjective criteria--such as by making the transition from legal to illegal sufficiently smooth in terms of possible punishment that the punishment statistically scales with the probability of the sexual interaction being with an immature person: While that obviously does not perfectly match the consequences to the actual maturity of each individual, it would give a much better correspondence between punishment and the abusiveness of the relationship overall than a hard cut-off, while still being based on objective criteria.
Now, you are right that the law is actually usually more complicated than a single hard cut-off, but it seems that it's often still doing a poor job due to lack of nuance.
As it happens, statutory rape law is already more complicated than a cutoff at 18. There are often exemptions for people who are married, or close in age. But the line is still objective — a “bright and clear line”, so to speak.