They're still physical way underdeveloped and it does not mean they can consent
Even mainstream opinion
now holds that minors under the age of 12 can give informed consent to life-changing procedures, including
sex change treatments, but informed consent will always be an arbitrary concept. Both children and adults often give uninformed and informed consent to sexual and non-sexual activities.
With respect to sex, even assuming current social stigmas, simple consent (yes/no) is enough to predict that long-term outcomes will likely be neutral or positive— this is backed up by many studies that asked adults how they felt about their childhood sexual experiences.
Consensual sex is not like physical combat: if differences in physical or mental abilities automatically led to exploitation, most adult relationships would be exploitative because adult men are generally psychologically and physically stronger than women.
A child can get an adult man jailed just by reporting to a random person on the street that he had sex with them. This actually gives children a disproportionate amount of social power over adults, especially considering the fact that most children know that pedophilia is illegal and could easily use that knowledge to harm or take advantage of their adult partners.
or understand the weight of sexual situations.
This is circular logic. If a person does not fully grasp the consequences of something, you educate them. However, to provide a child with the required information is called grooming which is also looked down on. If they don't know, you inform them but the act of informing is also deemed bad.