We may use the equation:
A = A₀exp(-k*t)
Where k is the radioactive decay constant and related to half-life as:
t(half) = ln(2)/k
Substituting k from the second equation, and putting the values:
14.2 = 15.3 * exp(-ln(2)* t / 5,730)
t = 617 years
The artifact is 617 years old, which means it is from 1371.