JF1980
In Hong Kong it is illegal for companies to use bonds to tie down employees for any amount of time. As far as I know, when you join as a cadet you sign a training contract, and before you join CX you sign a "Conditions of Service". I'm not quite sure about the exact contents/ramifications of these documents, but I don't think they legally bind you to CX for any specific timeframe. Therefore you are free to go when you please. Perhaps someone actually in the company could fill us in on these details.
Being 30 is fine, I have seen cadets who are older than that. If you can prove to them you are just as passionate and suited to a flying career as all the younger guys, you can make the cut.