Though I agree with you,
Cognitive impairment is relative, wouldn't you say,
What with some adults being uneducated, poorly developed by nutrition, or having psychological bents,
Feels a gray sliding scale area.
That someone as not as capable, does not follow they're 'incapable.
Compare an average man shooting basketball hoops with a professional basketball player,
Though the average man is poor in comparison, he's still able to shoot the ball through the hoop some of the time.
It feels more a social/cultural/definition based idea, consent.
What with older societies having kids start earlier in life, what with war and short life spans. . .
By such I'd still say children can't consent, but it's a. . subtly understood definition, a social norm with reasonings behind it, we don't really bother spelling out, though they're there.