The difference between ages of any two members is 3.5 years, we can assume the ages to be x, (x + 3.5), (x + 7), (x + 10.5) and (x + 14) years (Note: Among these x+14 is the eldest).
It is given, x + (x + 3.5) + (x + 7) + (x + 10.5) + (x + 14) = 145
5x + 35 = 145
5x = 145-35 = 110
x = 110/5 = 22
Age of the eldest one = x+14 = 22+14 = 36.