I adopted a senior German Shepherd last year. Lost her to GDV three months later. Gorgeous dog, very friendly. Still up for romping in the field and long walks. I was heartbroken when she died. I adopted a two year old half a year later. While I don't think I will never adopt a senior again, it was still quite a shock too loose her so quickly.
Not only that, vet costs were through the roof with her. I probably spent more on her ($8000 in three months) then I have spent on my other dog in 10 years. Of course, this also depends on the breed and condition of the dog you get. My GSD was a puppy mill dog before we got her, so her genetics were fairly poor. She had hip dysplasia, broken bone injuries that healed poorly, very poor teeth, and also chronic eye infections.
Just something to consider... I know it is different with every dog, just keep it in mind. Old dogs always seem very thankful when they get adopted, and most are much better with young children then some other young dogs. Many old dogs don't find a home, so those that do are very lucky. Possibly after my current senior passes, I might consider another older dog.