The age of consent exists to protect society from the predictable consequences of relations between older men and young girls.

When these kinds of relationships become common, violence follows like white follows rice. Fathers and brothers attack men who do shit like this, OR in the worst-case scenario, fathers and brothers become men like this themselves. Because young girls are not sophisticated enough to recognize the dangers (not a matter of intelligence, just lack of practical experience which comes with more years lived), relationships between this age group and older men reliably create major social friction. That's why we have age of consent laws.