Seniority of merged partnership firm

Actually, it will depend on the name of the new firm that you are establishing. ICAI has released rules for the same and I'll advice you to have a look at them.

Following are the scenarios possible:

1. Name of New Firm: AB & Co. - In such cases the date of establishment regarding senority will be from date of old senior firm i.e. from 2010.
2. Name of New Firm: XYZ&Co - It will be from the date of registration of the new firm irrespective of the senority of old firms i.e. 2018
3. Name of new Firm: A & Co or B & Co - Then it will be their respective date i.e. 2010 or 2016

Also, you would be required to file an application to the ICAI for the approval of the same.