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I agree with @Tgotty90. A rough guide for aging a new world immature tarantula that maxes out at 6 inches DLS( and I mean ROUGH since some are fast growers like CBB and some are very slow growers like B. Hamorii) is to guess the DLS in inches and assign 1 year per inch for a female and 1.25 inches for a male. So, if your Rosie is 4 inches DLS, she is somewhere around 4 years for a female and 3 years for a male--give or take a year.
i don't think there's any real way to measure years, it all depends on their growth, and that is all determined by feeding schedule, temp, humidity, etc. unless you know the exact date of the hatch of course