I don't think there is a fixed answer for this, but "Too old" to me means that the cat is too old to look after itself properly, or is too old to do certain activities.
Not at all. Surprisingly, many people thinks this. I had to set a friend's mother straight years ago, because she thought her 10 year old cat was so old she'd have to put her to sleep soon. Last...
Keep them in the same room as you, so they get used to each other's presence. Give each one an equal amount of attiantion, so neither is jealus. After a while you can move them closer and closer to...