I am not sure if this is a cosmetic damage or a structural damage in the tire. The tire is a Bridgestone Turanza 225/40R18 in front. About 18k miles and almost 5 years old. I don't know where I got this cut. Should I replace this immediately?
There seems to be metal poking out of the cut, although it could be a trick of the light in the picture. If it is metal then the metal sidewall reinforcement is damaged and the tire's integrity is compromised, in which case you should replace it right away.
From other angles it looks like the glint is from some sort of wire buried to give the tire a little ridge, in which case the integrity may or may not still be good. However, that split is pretty big, I personally wouldn't feel comfortable driving on that.
Since the tire looks like it doesn't have a lot of tread left, and 5 years is getting on for a tire on balance of factors you should replace it.
You are lucky to have gotten 18K miles and 5 years service. Even if this tire was new, REPLACE right away. I wouldn't drive this tire on expressways, keep it under 35mph until you get it replaced.
It's not worth taking the chance with your safety. 55 years experience talking.
I work in a make ready department for a car dealership, and we would absolutely always replace this on a stock or customer's vehicle. While perhaps not "dangerous" to drive to a shop, it's going to blow out before too long, and repairs to sidewall cuts or punctures never last very long.
So, short answer, replace it now, don't try to repair it or drive on it for any distance.