ive a patterdale , lakey cross here, at 3.5 months old he caught and killed a mouse on his own down at the river i fish, i kid you not, from that he moved onto earth work himself and now hes hard at his work and never looked back, rats are just a bit of off season fun for him. id say if any terrier refuses rats or squirrels then he aint no terrier. i took mine to the moira game fair this summer and about 100 yrds away from the ferret stand he smelt them and was cart wheeling on his lead to get at them the we shit lol. some have it and some dont. but your dog is young and prob given squirrel