Definitely before 25 - while you still know everything!
Best age is now.
If you have rich parents, then starting it when you're younger makes sense. When the startup succeeds, you win. When the startup fails horribly you use that story to up level far higher in a big organization than you otherwise would. Make sure you nail the rich parents part. See also the CEO of Indeed.
What if I dont have an idea
Rich parents is the most important part. Dumb idea is fine.
10% odds of success and .01% chance of unicorn regardless of when you start. Put another way, you’re 90% likely to shut down in 2 years and 99.99% likely to never reach unicorn status ever. Now the experience of founding a startup maybe worth it regardless of whether you fail or not. Up to you to decide on that, I’d say it was worth it for me even with a -$23k exit. MBAs cost more than that and probably don’t provide as much education into business as founding a startup does.