What I understand is that, as per the agreement of the merger, all the benefits previously admissible to the employees in the organization that was merged into yours need to be protected. This means you need to maintain the status quo regarding their benefits.
In this context, please let me know whether the terms of the merger agreement were accepted by the employees or the union. If accepted, you can retain their retirement age at 58 years, and they cannot agitate over this issue as you have not done anything that adversely affects their service conditions, and you have complied with the terms of the agreement which were accepted by them. If not, it is advisable to fix their retirement age at 60 years, on par with the employees of your company.
Regards,
B. S. Aikumar
Mumbai