There are no general rules on how to apply for a PhD programme. Furthermore, application requirements and procedures tend to vary from programme to programme.
Generally there are no age limit for starting a PhD in Germany. It rather depends on the date of your last academic degree. If your last academic degree is older than six years, usually you are not any longer eligible for many funding programmes. Exceptions can be made because of e.g. maternity leave, sick leave to care for a family member or a serious medical condition that occurred during the first six years after your last academic degree.
In summary, advanced age or the possession of another PhD degree are no criteria for exclusion from PhD programmes. The most important formal requirement is a master's degree (or equivalent) that is recognised in Germany. Other important factors include your work experience and motivation.
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