Their differences may eventually stop the separated squirrels from breeding with one another this is an example of

Their differences may eventually stop the separated squirrels from breeding with one another this is an example of  an Allopatric speciation

Explanation:

In allopatric speciation, species from the same population evolve some different characters due to the differences in geographical locations.

In this scenario, the species become reproductively isolated and diverge.

The mating times of the squirrel have changed due to the environmental changes.

They no longer interbreed and hence no viable offspring thus sexually isolated from each other.

age range would be the answer