Google brought in veterans like Omid early on. However, I believe Facebook waited much longer.
It's never too early to bring in experienced people. At an early stage company you're operating with limited resources and you can't afford to waste time making other people's mistakes again.
Bring in experienced people as soon as possible. If you think you can't afford them, try to find people that will share their expertise with you for free.
It really depends on your startup and which stage your at. Its something that you need to feel out. I've learned the hard way that you may not even need experienced people. Feel free to reach out and we can discuss it further.
Experienced people are the first to hire. younger inexperienced employees will benefit more with their mentorship. Your company will benefit from people that know exactly what to do to succeed. why would you put your start-up in the hands of someone who never had the experience? i am not saying don't hire any junior talent but get the right experienced talent to inspire and motivate them.