It'll be slightly strange entering any field at 31 when many others are getting started at 21. Other people your age will have ten years of experience and you'll be still getting the hang of the ropes for the first time, but unless you have a problem with that, no one else should either. :) The reason people in the book are 5-7 years younger is that it paints a more romantic picture, not because there aren't older people there getting equally successful. To the extent that young people succeed where older people don't, it's because younger people tend to have fewer obligations and a work-life balance more stilted towards work.

If you're going out to work as an employee, even at a startup, it's unlikely that you're going to make it big time, but it's fairly likely that you can make it an interesting, positive and lucrative five to seven years. (If 10% of startups are successful and it takes 2-3 years to figure out if a startup has succeeded or failed, then you'd only get a 20-40% chance of being in a successful startup, and even then employees are unlikely to cash out in a big way in a short timeframe.)

In this case, with low risk comes medium reward. :)