There’s no age-exclusive related safety concerns. Generally speaking, if both rabbits are relatively healthy and energetic, the safety considerations will be the same as with any two rabbits, so watching to see how they communicate and if either are aggressive, being mindful of how severe that aggression is and having preparations to handle and break up those interactions immediately.

And yes, the passing of a bondmate is always something to consider and is ultimately an inevitability with how life works… when the time comes, it is up to the owner and rabbit to see how each other is doing and go from there. Every rabbit reacts differently to the passing of a bondmate. It can seem like they don’t care at all or it may impair their health, in which case the owner should work to find out how to help the rabbit through the grieving process.

It is something to keep in mind considering how large the age gap is, and also understanding that it often takes a couple weeks to months to bond as well. If they are enjoying each other’s company side-by-side pen living, that can still be beneficial and provide social stimulation without the formal bonding. I actually have two single rabbits who interact through bars safely, but they are not compatible as bondmates. Both still seem to benefit from each other’s presence though.

As long as Rex is cleared as healthy then I think you are OK to give it a shot. You’ll just want to go slowly with the bonding process and definitely go at Rex’s pace.

The thing with one bun passing away is basically always a concern. Even if they are close in age, one will always be left alone, so in some senses it’s easier to have the younger bunny left alone because if Rex was left alone again he’d need to go through the bonding process all over again at an even older age.

I would just see how they do on some short dates in neutral territory. If you get a sense the bonding will be very long and stressful, it may not be worth continuing.