Honestly, I'm not 100% sure. It might be his 40th birthday. But to be certain, you need to re-read the relevant parts of the novel and look for clear indications.
Well, it could be his 35th birthday. However, it depends on various factors and details within the novel's context. Maybe you should go back and look for specific clues to get the exact answer.
I think it's his 30th birthday. But you'd better double-check in the novel to be sure.