There are studies that show that the overwhelming majority of tubal reversals are sought by women who have decided to have another child. The reasons are usually that a child has passed on, or the woman has remarried and wishes to have more children with her new husband. I don’t know of any studies done that could differentiate between those who will always wish to remain childfree and those who will change their minds. I wouldn’t even know how to begin to differentiate between those two groups, and you probably can’t.
[nitpick]
I knew at 14 I wanted to be sterilised one day. I’ve always known, since I knew that humans made children and didn’t pick them up at the grocery store when they wanted one, that I didn’t ever want to make my own. 
[/nitpick]
Tell me Primaflora, you say you didn’t want children at 18, and I believe you. But did you seek out sterilisation ? Did you know the details of the different procedures available ? Did you seriously save and plan towards it ? Or did you just think that children weren’t for you ? I do honestly think that many people change their minds, but they aren’t likely the sort who have spent a long time researching permanent options. Not that I’m saying all women who get sterilised never regret it, some definitely do. But a lot of people regret getting tattoos for example and we don’t make them wait until they’ve had three temporary tatts and are over the age of 35, do we ?
(yes, hyperbole, I know!)
BTW, I’m not distressed psychologically by the idea of babies, I just really, really don’t want children of my own. Yes, it’s not a ‘normal’ view, if you take normal to mean the usual, or the norm. There’s nothing wrong with my mind, I just don’t want children. Someone else could feel the same way about birds, and all they’d have to do is not go to a pet shop and buy one, I take a risk (granted, a small one) each time I make love to Mr Goo. I wish it came down to me just refraining from making a purchase… if only it was that simple 
As for those struggling with fertility issues, yes, they have my sympathy. I would hate to want something so desperately and be unable to get it, I can imagine the inability consuming your life and making you hate your own body for not cooperating. I can see it tearing marriages apart and ruining lives. But for those in that position colely because they voluntarily sterilised themselves and then changed their minds… well, what can I say ? I don’t think anybody is sterilised these days without knowing that it is permanent, so they’ve kinda only got themselves to blame. If I ever get a yearning for kids, I’ll have to satisfy it by fostering, having expensive treatments, adopting, volunteering, or just get on with the consequences of the life decisions I made. I don’t think we have to ‘protect’ people from the possibility they may regret their actions later, I think people should be made responsible for their actions.
On a re-read of your post, I don’t think we’re poles apart on this topic, in fact, I would be surprised if we didn’t agree on most aspects. I know you’re a fan of personal responsibility, even if you’ve seen the heartbreak of infertility up close. I know you wouldn’t support anything that would prevent me from having the procedure done as soon as I can afford it.
I personally don’t think you can do anything but set the age at 18, at least not here in Australia. I mean, you’re either a legally responsible adult or you’re not, though I would understand a doctor’s objections to carrying out the procedure on an 18 year old. What would you prefer to see, Primaflora ? 25 and above, after writing a logical, though-out essay on your reasons why you wish to get spayed ? SO’s consent ? 22 if you’re really really persuasive ? I’d be interested on when you think it’s reasonable, since I just can’t see any reasonable cut-off other than 18 / legal maturity.