[BBury]

You can create contrails at any altitude.

The contrail altitude starts, as far as I can remember off the top of my head where the outside air temperature is -37 Celcius in ICAO, ISA conditions. (International Aviation Administration Organization, International Standard Atmosphere)
So it would be variable depending on what map you're flying.
To check this in game would involve taking a bomber that has a OAT instrument up to the altitude where it is -37C and then determining from there.

There's no standard altitude in the real world for contrails, but as a gerneral reference the ISA should be used, as that is set as the average atmospheric conditions worldwide.

Exactly! So on a ISA map with standard lapse rate, the contrails would start at 30,500ft.