I have a 8 month rabbit (male, neutered) and I'm considering getting another one so they can play together. What gender/age should I go for?
The important thing in building bonds between rabbits is the character of the rabbits. An extreme boss-like rabbit would not accept another boss. But also an extremely shy rabbit is not good as pair for a boss, because the boss may act too agressive without counterpart (like not allowing the shy to eat/chill).
The most rabbits are not extreme. Two of them would give a good pair. Easiest pairing would be a neutered male and a female. This is in most cases stressless, because nature made rabbits liking to live as family.
If you would give them a favor, then you choose a rabbit that is almost the same age. Then they have the same interest like be very playful when they are children, or lay in the sun when they are seniors.
Here is a picture about the "most times average outcome" of pairing. (Traffic light outcome: red = not good, yellow = half and half, green = good preconditions, pink rabbit = female, blue rabbit = neutered male.)
To plan the first meeting is important and will affect the outcome! How to bring them together you could read here, when you click the tag "bonding".
How I learn it (and how it worked for my three new pairings, each time a male with a female) is this:
You need a space both rabbits have not used until now. This is because rabbits mark "their" area and would defend it against new ones. In nature between rabbit warrens there is a neutral space. If rabbits do meet there, both of them do not need to defend it. They could get in contact. The area needs to be big enough that they could avoid contact, but small enough, that they do not need to search each other. In my case this was a plane and empty space seized between a one-person bed and a two-person bed (but not in the bed - on the ground) with a 1 meter fence of stable mesh wire around it.
All three times there were two interested rabbits that go to each other and sniff, sometimes one stuck the nose into the others fur, sometimes running a little dostance away. But in general both are curious and interested in each other. One time this was all what happened, the next step was they groomed each other. This is the best what could happen. But the other two times one (in both cases it was a female) wanted to rule the other. This looks like one do not like to have the other one near, and drives it out of its personal space. If the ruled rabbit (in my both case the male) respect the gap, then the "ruling" one do not fight/drive it away anymore. My females liked to dictate when they like to have the male near, and when not. My males do act like smart men, and accept the women's rules. Then we brought both of them in the normal rabbit surrounding and all was fine. (The first almost 3 days they had little differences about "How much is enough space" once or twice a day, but only for 2 minutes). In 2 cases both groomed each other the same day, in the third they needed 2 days before sitting together grooming. You should have a close eye on them in the first couple of days.
You should not separate them again after a good start. (Some people do it, because they like to "pair them slowly") Then they need to make the ranking new, each time you let them meet again. This could be frustrating, and in the end they would see each other as disturbing neighbours. Separation is only needed, if they do fight hard (biting in ears and paws) or if one does not allow the other to eat, rest or poo/pee. Then you need experienced help, or use the "worst case" option to bring the new one back to the breeder/shelter. A good breeder/shelter knows about that possibility and does not deny to take a rabbit back, if the pairing does not work. Instead they would help you to pair them, or even let you bring your old one in, to choose the new one.
Age doesn't matter too much. Usually male/female pairs are best, so a neutered female.