It might be because females are known to completely developed at 18 years while males need more time; 21 years.
Thats very interesting .....I would like to know why the age difference too!!!
I believe it is the culture , children listen to stories from parents , get inspired ,and the cycle moves on
I support future surgeon puberty in girls start at 9 to 10 or even at 8 while it starts in 13 in males. By 18 females can reproduce and are considered adults while it's little bit higher for males