Hello akhil,

There are various posts in merchant Navy like deck officer, engineer, catering department. Which post you want to join. The minimum age is 17 yrs to join merchant Navy and the maximum age is 25 years to give exams to join merchant Navy. 

I will provide you a link below which has detailed information about different merchant navy posts in india. Hope it will help and guide you

https://www.marineinsight.com/careers-2/joining-merchant-navy-class-12th-india/amp/

All the best