rats moult throughout life, the most visually drastic typically occurs around 6 to 8 weeks but often a fair amount of fluff remains until just before 3rd month. By third month most all of that "baby fluff" is gone (in standard coats of course)

This is just a statement of what I have witnessed & not saying anyone is more or less correct. Some rats go through a drastic moult as early as 4 weeks & others go through that patchy period where most people think they have some sort of special tri-color rat as late as 3 to 4 months.

The weight of rat would help determine age best, but even then...

At the end of the day... there is no recognizable size comparison. The person holding the rat could have hands the size of The Hulk or smaller than a 10 year old.

Who knows?