Our new puppy (lab mix) 5 months, has started attacking our 14 yr.old lab mix when I am close to both of them. The puppy is a sweat heart except for this. We also have a 3 yr old lab that the pup is fine with. Is this a alpha thing?

It could be an alpha type thing, or it could just be playful puppy behaviour. Don't separate them though, the older dog needs to stand up for himself and separating them won't change anything. Also, once the puppy is fixed, his behvior might change.