In Western culture, what age is typically considered to be the beginning of young adulthood?

According to the article Who's Old First about physical declines in old age, Shulz and colleagues studied baseball players by:

(a) watching them on the field (b) interviewing them (c) having players describe other players (d) a baseball encyclopedia

Which of these is NOT one of the three main factors affecting the child's mastery of motivation/achievement?

a. Having parents who apply authoritative parenting b. Living in a home with a stimulating environment c. Having an entity view of ability d. Being securely attached to parents