In my opinion, a 14 yr old dog does not need to put up with puppy behavior. I had my deposit for Seger in the mailbox the night before we lost our senior so I didn't experience it but our thoughts were to keep them separated just so she didn't have to deal with it. I think you should redirect and engage the puppy as soon as she starts. Laying next to your senior is fine. Playing gently is fine. Any rough play should immediately get redirected.