I've had to answer this question quite a lot lately. I guess the pressures on women aren't really letting up are they? I think maybe this whole issue is made worse by the fact that there's so much stigma surrounding women who can't but do want children that it's difficult to accept the choices of women who don't. But to answer your questions...

- I think honesty is usually the best policy. Not wanting children doesn't mean you don't like them (or can't act like you like them). If you can help out with your friends children this reinforces the fact you're not 'abnormal' or just being 'selfish' - and underlines that your decision is your own but you're still a good person and friend.
- I've always known i don't want children - to me it's as natural a feeling as those who do want them. Do i worry about growing old alone? Not really - children don't guarantee that sort of fulfillment. I don't currently have a partner, i was engaged to a woman but it ended quite badly. I think it's because the person i genuinely love can't be mine for various reasons. A lot of people would feel positively ill without a significant other AND children but it is what it is. I'm not a conventional person and didn't expect a conventional sort of life.
- I am at peace with myself, i've not had the best of luck or the easiest life but i'm resigned to my limitations. Fighting against it all is just a waste of energy. I feel fortunate that i'm not living in a time where women are destined for only one thing and should they fail that they're a disgrace to womanhood the world over.

Just be yourself, the promise of marriage or children etc can be a very hollow one, the amount of people i know who regret their decisions and can't even discuss it openly beggars belief. I know which sort of situation i'd rather be in.