The characters you play have to be exceptional in some way. Plus they are pretty young for the positions they hold in their careers. Capcom explaination is that they are academically advanced for their age and hence why they are in that position now.
like every character was so damn brilliant they did something at a surprisingly young age.
William Birkin: Birkin joined the Umbrella Corporation in 1977 at the young age of sixteen.
Rebecca Chambers: A talented girl, Rebecca graduated college at the young age of 18.
Alexia Ashford: her exceptional knowledge allowed her to finish university at the age of ten
Excella Gione: Excella quickly breezed through school and enrolled in a university at a startling young age
Carla Radames: Carla Radames was a highly-talented intellectual and began to familiarize herself with genetics work, finishing a doctorate by the time she was only fifteen years old
... only one of them is a guy, too... Seems like a japanese thing where all the women have to be young and nubile, but they need to be smart for plot reasons so....
And every one of them is a villian too except for Rebecca. And Chris who kicks every one of their asses is the stereotypical muscled dumb guy (he says he doesn't care what Wesker is up to in 5 just as long as he stops him).