Hi there. 이 is not the subject marking particle here. It is a linking element that is often in casual/low-formal speech to link a noun ending with a 받침 (i.e. a consonant) and the particle. The construction then is 수철 + 이 (linking element) + 는 (topic marking particle). Hope that helped.

수철이는 열 네 살입니다. In the phrase "수철-이-는" the suffix '-이' is used for making its pronuncation more easy. That's an epenthesis vowel. And this suffix '-이' is used only when addressing one's frirends or inferior. You should not use it for your elder or superior. 수철이는 열네 살입니다. 수철은 열네 살 입니다. For examples, 갑순, 갑돌, 춘향, 향단, 수철 갑순-이, 갑돌-이, 춘향-이, 향단-이, 수철-이