4. Can I apply if I become 31 before the last day of publication of job openings?

Your application will not be taken into consideration. Indeed, you may not be 31 on the last day of publication of job openings.

For instance, if job openings are published from 27 May to 5 June and your 31st anniversary is on 30 May, you cannot apply.
If, however, your 31st anniversary is on 6 June, you can apply for a last time.