It's not going to be a MASSIVE issue but it is pretty big. I had to get mine at 10 weeks because his breeder had some serious health issues and was being admitted into the hospital. It leaves you with a puppy that hasn't fully learned bite inhibition so he thinks that he needs to chew on everything. He also won't know where to pee or when he needs to go, other than when it just hits him. He'll still have the issues with holding poop and stopping eating because the sphincter in his bowels will not have finished maturing yet - not that at 12 weeks it's finished, either, but it is closer. He'll still have some food issues - I have to water down or milk down his kibble so he can eat it.
Those are only SOME of the issues I'm dealing with and mine is two weeks older than yours.
Oh, get him neutered. It'll keep him from lifting his leg and marking all over your house and/or anything he wants. It'll keep him being your baby and not a 'working dog' that wants to go after every female... There are so many reasons to neuter. ALSO, it lowers the chance of him getting testicular cancer.
Good luck and welcome to YT.
__________________ Jessica  Momma and personal assistant to Fynnlie |