If you mean writing an article you can begin as soon as you like. BTW . . . writing at HubPages is not a 'formal' job. Earnings are through revenue sharing. To learn more check out this link to the Facts and Questions (FAQ) for HubPages where many questions can be answered.
How to make money at HubPages: https://hubpages.com/faq/#royalties
The link to the FAQ is: https://hubpages.com/faq/