2-4 years is ideal I think. I want them to be close enough in age to be able to enjoy each other. Lucky was 11 when I got Rowan and he really hated having another dog around, even though Rowan was always very respectful. He didn't like other dogs much anyway but I think he would've adapted better when he was younger.
Rowan is about 5 years older than Riff. He was 6 1/2 when I brought Riff home. I really wanted to get a second dog before Rowan got older (8-9+) and it's worked pretty well. I want another while Rowan is still around, but I don't want him to be too old, and it might not be feasible until he's a senior. At that point I might just hold off. We'll see how it works out.
Kendall & the Red Dogs: Rowan & Riff