Let A(0) denote the amount present at t = 0.

Since the half-life is 4.5 billion years we have:

A(4.5) = (1/2)A(0)

Using the formula A(t) = A(0) e^(kt) we have:

A(0)e^(k*4.5) = (1/2)A(0)

e^(4.5k) = 1/2

k = (1/4.5)ln(1/2)

Now we need to find t such that 

A(t) = 0.9582A(0)

0.9582A(0) = A(0) e^(t*((1/4.5)ln(1/2)))

e^(t*((1/4.5)ln(1/2))) = 0.9582

(1/2)*e^(t/4.5) = 0.9582

e^(t/4.5) = 0.9582/0.5

t/4.5 = ln(0.9582/0.5)

t = 4.5*ln(0.9582/0.5)

t = 2.927 billion years

The age of the crop is 2.927 billion years.