In my mind there is a very distinct difference between pulling an older dog from rescue or the shelter, and purposefully choosing to purchase a dog that you won't receive until he's an adult because someone else is training him/her. If I had more money, time and space I wouldn't hesitate to rescue a senior I liked, but the difference is that I wouldn't have had the choice to have taken him or her when they were younger so I could enjoy those years too. It's not that taking in an older animal isn't rewarding and worth it, it's just that given a choice I would rather receive said dog earlier in life rather than later. This is not to say that some people may find buying a trained dog to be a good option for them, I can think of a few people off the top of my head that would totally go for that. But for me, if I was given the option of buying a blank slate or a pre-programmed dog, I would hands down take the blank slate.