As someone who loves training their rats everything from spin to fetch to painting an more, I'll tell you that there is no real age limit for when to start. This past summer I got some tiny 4 week olds (WAY too young but they were feeder babies) and started training right away. I started right away as they were confident, friendly, and treat driven so there was no reason to wait.
Now just a few weeks back I adopted a neutered 1.5 year old male rat. With him I waited a few days to start training because he was less confident and took a while to warm up to me. Once he did though we were able to start training no issue, and he quickly learned some of the basics and is close to mastering fetch!
The oldest rat I've trained was over 2.5 years old and relished it, always asking me to train each nigh during playtime.
The key is really just to start slow but make sure to always progress, don't make the mistake I did when I first started of drilling one thing over and over as then its hard to go forwards (my star rat Shadow took a WEEK of several sessions to learn spin because I kept using the lure instead of fading it out...on the other hand my 4 week old babies with no attention span at all learned spin in under 2 minutes because I now know how to quickly fade out a lure and introduce a cue).
Anyways the way I'd recommend starting training is starting with luring. Work up to them following a treat around by rewarding them after one step, then 2, then 3, and so on until they can do 5 steps before needing a reward.
Now teach spin using the luring method, then paw, and then I like to clicker train them. After that I teach backup, jumping over obstacles/to hand, nose target, fetch, and basketball. These are the core basics and once your rats knows these (plus pull on a rope but I usually teach that later down the line) they can pretty much combine them to learn any trick. (For example painting sounds complicated but its really just basketball combined with targeting a canvas - still a multi-step trick but not too hard to teach once they have the basics down).
Finally I run a youtube rat trick channel with tons of tutorials that might help you. Here is my video on the "basics" of rat training:
And here is my video showing how to train 10 "core" (beginner) tricks: