They can be done at any age really, boys need their testicles to descend before their able to be done. However when it comes to vets quoting you ages, that's more about their comfort level. Younger animals can certainly be done, it DOES NOT affect that animals growth.
If you want to keep your 2 together, they should be done around 3 months of age. If you wait any longer they will need to be separated as the male will be able to get the female pregnant. Then there is definitely no guarantee they will even tolerate being around each other again once separated.
Flemish being larger rabbits, it's much easier for vets to do them at a younger age.
My 2 are small breeds and male and female were done at 3-3.5months old for the exact same reason you're looking for. Brother and sister I did not want them to breed, and did NOT want to separate them as this can break their bond.
A competent vet should have zero issues doing rabbits this young.