Most startups don't fail because the team couldn't build. They fail because they built something based on an assumption no one tested. Your experience is valuable, but it is also a set of assumptions formed in a different market, at a different time, often before AI changed the economics of the problem.
The most useful habit a mentor can reinforce is: “How do you know?” How do you know customers want this? How do you know they'll pay that price? How do you know this channel works? If the answer is “I think so”, the next step is to find out, cheaply and quickly.
AI-native founders can run experiments faster than ever: a landing page in an afternoon, a prototype in a day, twenty customer conversations in a week. Help them design the smallest test that would change their mind, and hold them to running it.