Which stage do you think is best when a startup should start tracking goals (OKRs)?

As soon as want to have clarity and alignment on what you want to achieve and a way of measuring progress. So in my view Day 0! You'll probably won't get them right, but the sooner you start learning, the sooner you start improving! I've actually founded a company to help people succeed with (vs use) OKRs - let me know if you'd like to chat!