You have clearly ignored "aad". It is Adashta Varsha not ashta varsha. In Rigveda,
Rig Veda 10.85.40-41 “Soma obtained her first of all; next the Gandharva was her lord. Agni was thy third husband: now one born of woman is thy fourth. Soma to the Gandharva, and to Agni the Gandharva gave: And Agni hath bestowed on me riches and sons and this my spouse.”
Now read padma purana,
Padma Purana V.118.2-15 “..When small hair appears (on the private parts), Soma enjoys a daughter. When she attains puberty, gandharvas (enjoy her); and when the breasts appear (prominent), Fire (enjoys her)…”
When prominent breasts appear only then she gets fourth husband. After periods starts, it takes atleast 2-4 years for proper breasts to appear. So, it will be around 15-16 years. Also Rigveda comes before any holy scripture.