It is possible to spay the female at any age, but the ages mentioned are the most convenient.
"Common Diseases of Farm Animals"
R. A. Craig, D. V. M.
Is this a farm to you, Where for all woods I spay one tuft of rue, And that so rusty, and so small a thing, One shrill cicada hides it with a wing; Where one cucumber covers all the plain; And where one serpent rings himself in vain To enter wholly; and a single snail Eats all and exit fasting to the pool?
"New-Poems"
Stevenson, Robert Louis