They don't hit puberty when they're 18/20. They hit puberty when they become teenagers at around 13. It states it explicitly that the "they can't tell" phase is before puberty. But ya'll want to make it as if they reach 18/20 and they still can't tell their age.

Male Viera sees a teenage boy in their village and they're taking them. They're not waiting until they're a full grown man at 20.
Do you all even follow the lore?