If you are walking the pup over ground with any cover ,brambles,gorse, nettle's, heather,etc,the dog will naturally want to investigate.As for age,there's no hard and fast rule's,some dog's are game earlier than others,usually at least 6 month's before you should show them a rat.Seen young dog's ruined by being thrown in at the deep end to early,one good bite in the wrong place,and the dog might throw the towel in before he's even started.hth,Bill