Hi, my car is a 2009 with only 19 thousand miles. Coolant still the original from car was made. Is it time to replace it? Of so what's the easiest way to do it? Just drain from radiator and flush with distilled water a few times? Coolant life goes more by the miles or age?

Did you read the owners manual?

I just did this after replacing the thermostat.

easy to drain from the red petcock at the bottom of the radiator.

I did not do a distilled water flush but you can if you feel like your system is contaminated.

refill with a 50:50 mix of MB coolant and distilled water. There is no bleeder on the system - I had to go through several cycles of bringing the car up to temp, letting it cool, and topping off fluid before I got all the air out.

Ok thanks, i didn't feel comfortable to leave 13 years old coolant in there. I'm gonna do it this simple way and do it every 2 or 3 years. As i hear it helps preserve the headbolt from rusting(untill i change them)

Mercedes specifies that the coolant needs to be replaced every two years. If you have never done yours, it has long ago lost its protective seal conditioning, anti-corrosion and anti-bacterial properties as well as its ability to effectively transfer heat and thus cool the engine.

The service booklet that came with my 2010 says to replace coolant every 240k km or 15 years.

This has pretty much been my method for any car I've ever owned. I do a full drain/refill every 2-3 years. The only time I would do a full flush would be if I bought a used car and didn't know what fluid was in there, if someone had put the wrong fluid, if I was replacing a component of the cooling system, or had some reason to believe the system was contaminated.

I stand corrected. On all turbocharged engines it is 30,000 miles or 2 years, however, I don't see the same for the NA motors.

Even on a non-turbo motor though, I'd still replace the coolant every three or four years because it does lose its ability to absorb and transfer heat just like it loses its seal conditioning and anti-oxidation properties.

What's even more insane, there's absolutely no service interval for the PS fluid.

the service interval is...wait til it whines...

Somebody once told me that if you change the coolant too often, you’ll lose some of the system efficiency or cooling properties. It’s 15yr coolant. If you don’t change out the overflow reservoir every time, which has an additive pack integrated into it, you’re not making things better, just worse.

I’ve also read that the packet in the tank is to extend the life of the coolant.

Either way, the coolant is designed to be used for a very long time. Don’t change it if you don’t absolutely need to.