My Luna is 4.5 months old. I've been trying to get her a friend for the past month. (I've had her for 2.5 months). Ideally I'd like to find a glider who is no older than 1yr. I'm meeting a 10 month old glider this weekend, whom I'm really excited about. I would assume you don't want more than a year between ages. I've never asked anyone experienced about that though. I just figured that within a 12 month age difference, there wouldn't be too much of a difference in temperance or size.
I would start looking NOW though. I've been on a serious hunt for a second glider for a month, and I still haven't found a suitable one. Sugar, the glider I'm meeting this weekend, and her owners seem very normal and promising. But it's taken me a month to even find a glider I feel MAYBE will be a good fit. So many people just stop replying to me, or seem really sketchy, or have sickly gliders, or want to ship me a glider (I insist on meeting a glider in person first). Plus you have to get a second cage all set up for quarantine and introductions, and that takes time. I would suggest starting the process now. Plus I know that even if I bring Sugar into our home, it will still be a solid month and a vet visit for each glider before the two can ever meet.