Old/new thing is not really true.
There's a common name, a systematic name and an IUPAC name (yes, IUPAC and systematic can be different).
The common name is generally used only for simple/common compounds such as ethylene. We say ethylene because it is easier to say than ethene.
The systematic name is often the same as the IUPAC name but can be different. It follows the naming schemes.
IUPAC is typically the standard when it comes to this stuff. Occasionally, it uses a more common name but typically, it follows the systematic name.
You should have a look at the carboxylic acids:
Common: acetic acid
Systematic: ethanoic acid
IUPAC: acetic acid