I have a 12 yr old yorkie. I got her when she was 7 yrs old as a rescue. She is pretty much healthy now. She's had her share of the usual problems like urinary tract infections and she needed her teeth pull. Because she is a senior, she gets a wellness check twice a year now but other than that there's nothing else special I do.
Make sure you steps for him if he will be sleeping in the bed with you. You don't want him jumping off the bed. Also, your 9 month old will still be very playful, the 12yr old....not so much. Just keep that in mind. Hopefully the two will get along.
Also Brandi is really laid back with the periodic burst of energy but she can keep up with me on a two mile walk. Okay well sometimes she demands that I pick her up when she gets tired of walking.