Auggie started classes when he was just over a year old and his first trial he was about two and a half.
Payton doesn't have a starting point because we were always working on "agility things," even if it wasn't around equipment and stuff like that or traditional foundation exercises, a lot of what went into Payton was working on having a great working relationship period. He was two years and two months at his first trial.
Georgie was about two and a half-ish. Her training isn't really clear-cut but "about a year" sounds right.
As far as having foundations down, well, the way I currently see it, you're never quite "done" with foundations. There's always stuff within foundation work that will make your dog a better competitor. Auggie was trained differently than the puppies; it was very much learn this, learn that, learn this obstacle, now start sequencing, longer and longer sequences, now run courses forever. Payton and Georgie do some sequencing, but still much of what we work on is what I would call foundations. Jumping skills are huge in our work. Weave pole entries. And for bad Payton we will probably forever have to work on his contact behavior. Sends to tunnels and jumps. It sounds really simple but even some of the more advanced stuff comes down to those. Before Louisville I worked with Payton on obstacle discrimination with tunnels underneath contacts and it was basically restrained sends. So I suppose once you know your foundations, it makes things easier in that you know how to train them and you can generalize behaviors rather than having to start from scratch and teach all new ones... but I don't personally feel like there's a point where you go "okay, we know all there is to know."
Agility is always changing too, creating new challenges you have to train through. International courses have backside jumps but you never used to see them in US style courses. Well, now judges are putting them in. So now people are going to have to either train those or figure out how to handle and make it happen. We had two backside jumps in the master's level courses down at Louisville. There was one at the Westminster agility show. It's clearly a thing now so expect all the Big Name US people to start coming out with articles and videos and junk on how to do backside jumps. ;P
So no, I don't think once you've got foundations down it's all smooth sailing, LOL. But it DOES make it easier than if you didn't have solid foundation work... if that makes sense.