So my Russian at 3 years and 5 inches will be a juvenile for 7 more years?There is no scientific answer to this question. Just what we end up calling tortoises of different ages/stages. Absolutely nothing set in stone, but... I think people who have raised a LOT of tortoises will pretty much refer to them this way:
A hatchling is from hatching to loss of egg tooth and starting good growth with visible growth seams (2 mos usually).
A baby is from that 2 mo (?) stage to 1 year old
A yearling is 1-2 years old
A juvenile is from 2 years to sexually mature.
Adult is sexually mature.