Will a 20-year-old be considered for admission into the ISI Kolkata BStat or are younger ones preferred more?

First of all, please do not worry about the age limit because as a 20-year old you'll definitely be considered for entrance exams. There is a rule which states that you cannot appear more than the fixed number of times for the same exam, but there's no rule which bars you because of your age. 

You'll find that you will have seniors in your batch too and maybe those with similar concerns, such as yours.

As for the admission process, you do not have to worry about looks, age, number of years, height, weight, or skin color. Except for your appearance and behavior, the rest is overlooked. Also, you must be fast at calculation and logic to do well in the interview.