Not that it is not an extension. If your visa is expiring, a whole new application would be required, providing all documentation again.
There is no age restriction on the subclass 482 visa.
You can transfer your current visa to a different employer/sponsor at any time, provided all requirements can be met.
If your intention is to apply for Permanent Residency, it may be difficult after the age of 45 as there are only limited exemptions available.