Those are 2 very very very different ideas you’re trying to compare, and feels like poor logic.
Teenagers can absolutely consent to sex, as sex and grooming are very different things. 2 teenagers having sex, normal. Someone much older than a teenager grooming them mentally for years to eventually have sex, not normal.
Lastly, elderly people’s mental faculties declining that hard isn’t guaranteed. Plenty of old people stay mentally sharp and capable of making decisions. Teenagers, though, 100% will have an under-developed brain until ~25, not to mention how little of life experience they’ll likely have.