If your intention is to bond, I think the easiest situation is to get an already spayed/neutered bunny, preferably one that you can have your bun meet in a “speed date” before adopting. If there is a shelter or rescue in your area, they would be my top recommendation.
If you get an unspayed female, it takes several months for hormones to settle post-spay, and you won’t be able to have a good sense of whether she will get along with your boy, as their behavior towards each other is different when they are hormonal.
The age itself is not so much of an issue in terms of the actual bond, and personality towards humans doesn’t reflect what they will be like with other bunnies.
What makes you think older bunnies would lack litter training? Or are you referring to getting an older, unspayed bunny, and then spaying?
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.