Most vets usually recommend 5-6 months, Ziggy my Avatar is having his nuts off next week at 4 and a half months, the vets were fine with it, the only down side they said to me is that he may be slightly smaller than the average cat, In America you can get them done at 8 weeks old, which I think is unfair, One of the reasons we want Ziggy done is that from an early age he gets very aroused by fleeces and his little member is permantly out through the whole process until he is finished, (wet patch left) Plus he cries at the dog flap because he can't understand how the other cats can go out and not him.
If you can check that both testicals are dropped, depending on age of kitty it should not be a problem.