Seder Hadorot brings different opinions as to how old Hevel was when he was killed by Kayin.
Bershit Rabbah Ch. 22: 50 days (or 90 days [Seder Hadorot says 50 is a printing mistake])
Ibn Ezra: 1 year
Tanchuma, Abarbanel and Josephus: 100 years.
According to the former opinions... does this mean that Kayin and Hevel developed really quickly or were they somehow born fully developed?