There is no reason why you can't pursue a postgraduate course in English language at the age of 55. In fact, it's a wonderful way to keep learning and expand your horizons.
Most postgraduate programs in English (like MA in English Literature, MA in Applied Linguistics) require a bachelor's degree in any discipline. However, some universities might have relaxation for mature students (above a certain age) with a relevant diploma.

Apply to universities offering MA in English or related programs. Explain your background and passion for English in your application essay. Highlight any relevant skills or experience you might have (e.g., communication skills from your engineering background).

Some universities offer postgraduate diploma or certificate programs in English specifically designed for working professionals or those seeking career change. These might be shorter and more flexible options.

• Consider universities known for their English language programs, such as Delhi University, Jawaharlal Nehru University (JNU), Central Universities, and reputable private universities.
  
  

• Look for universities with programs offering options for distance learning or part-time studies, which can be helpful for managing your time.
  
  I hope it helps!