MaxSize=500; MinAge=3; MaxAge=100;

F1[size_]=MinAge+(MaxAge-MinAge)*(1-(size/MaxSize))^2

Plot[F1[s],{s,1,MaxSize},AxesLabel->{"size","days"}]

F1[MaxSize]//N

F2[size_]=MinAge+(-MaxAge-MinAge)*(size/MaxSize-1)^3

Plot[F2[s],{s,1,MaxSize},AxesLabel->{"size","days"}]