[pxlpshr]

In their defense if they are active developers or entreps then they are likely extremely busy. As much as they'd love to help you, there are a lot of people like yourself asking for their time too, you're not the only one. So don't take their response too personally, you can likely try again later.

Egotists aside, I think you should evaluate their response and you might considering changing your approach if they are active and not retired. Being a mentor takes time, so think about ways for it to be mutual. Can you provide knowledge or assistance to their current goals before asking for it back? You may also try to show what you've done and that there is more substance to you than just an email talking about lofty ideas — ideas are a dime a dozen, it's all about execution.

Another general rule of thumb, don't send a 4-paragraph email professing your admiration or telling them things they probably already know, do your research. Sometimes this is received well, sometimes it is not. I had someone approach me recently, sent 3 paragraph email suggesting & instructing me how to embed an iPhone bookmark icon into a clients' website; I've been making iPhone apps for 8 months. I've made this mistake a few times trying to prove my smarts, use your Draft folder which is a good habit to get into anyway.

Sometimes you need to get in front of them. If you see them at a happy hour or developer meetup, approach them. Seeing people try is a lot more meaningful, it improves your odds and first impression. Twitter is great for this, follow people that interest you — they'll often RT developer meetups and speaking events.

Lastly, pick a mentor that compliments you and do your research. Finding a good fit should naturally improve your chances of connecting with them.