His birth date was "X". Today is "Y". "Age" is a measure of TIME from "X" to "Y". Nothing that his mother did to spike his bathwater changes either "X" or "Y". Thus "Age" is not stunted. His development was stunted, yes. Growth and development may be (in some ways) interchangable. Growth and age are not.

You might find the use of time as inaccurate to judge a person's "age". I do not.

Look, a larger world with the same rotational speed will have more "time" in a day. Even if they can divide it all evenly into 24 hours of 60 mins of 50 seconds each, those seconds would be longer. Then again, they could be shorter.

Some can go for the orbit of the planet around its sun. Years can be longer or shorter.

The variables you suggest are more variable than you want to comprehend. The measurement of time is subjective.

18 years of Terran time is measured by 18 rotations around Sol. Which is not the same as 18 years of Martian time, which holds a different orbit:

The solar day (or sol) on Mars is only slightly longer than an Earth day: 24 hours, 39 minutes, and 35.244 seconds. A Martian year is equal to 1.8809 Earth years, or 1 year, 320 days, and 18.2 hours.

A one year old Martian would be different from a one year old Terran... And we know nothing about Carna, but I will point out that probability tells us that it is

*extremely unlikely* that Carna will have exactly the same orbit around its sun as our planet does ours.