In the first one or two weeks of their training, I recommend implementing pair programming. Pairing your junior developers with more experienced developers or you. One actively coding while the other provides guidance and support, then switch places. This hands-on collaboration can accelerate a lot the learning process.

After that, the best place to continue learning is on code reviews in PRs. As @half0wl mentioned, it's importat to do it in both directions: you review their code, they review your code. This feedback loop will help your junior developers learn best practices and gain a better understanding of the codebase.

To facilitate these code review conversations, I recommend utilizing a tool like Pullpo.io (https://pullpo.io/products/channels), It allows you to have more enriching code reviews quickly on Slack, without wasting much time and being able to merge the PR sooner.

That said, learning curves can vary for individuals, and it's crucial to foster an environment of continuous learning and growth. But if after two years they can't keep up, i think it may not be the right person