Bella's love for her mythic true love is fictional: something so strong that it cannot exist in a rational world. No one like Edward is possible. None of our lovers, partners, husbands can ever be as perfect as Edward -- because Edward is a romantic fiction
Maybe in the end, we aren't going to be able to find proof that this love is worth the price that Bella will pay, because it's not something that can be demonstrated. And maybe it's a mistake to try, because the strength of that love is axiomatic. It's the premise on which the story stands (or falls). We can examine it here on the threads, and see how undersupported it is in places by the text. But as readers, we accept it, because that's what makes Twilight the story it is....
I think December
may have it the nail on the head on why I am having such a hard time speculating this.
I do think that, for the number of reasons stated, Bella is mature for her age. But to me, her age and speculative maturity level is somewhat irrelevant here, other than the fact that maturity itself makes it easier to believe she is capable of making of such a decision. Because, whether you like it or loathe it, as December
said, what we are being presented with here is an all consuming, out of this world devotional kind of love that does not exist in real life. So the fact that many teenagers (or adults) who marry later change their mind does not really matter. They can't be put on the same scale because this type of romantic love does not exist outside of the realm of fiction.
In regards to Bella's young age, I see a lot of "What if Bella is changed at 18, and then after being a vampire for a number of years she gets bored/misses being human/regrets changing for some reason?" thoughts around. And my instinctual thought is always the same - it doesn't matter what would happen because it just wouldn't.
And now I know why I think that. Because if it did happen it would detract - or perhaps even entirely cancel - the theme in these novels about out of this world romantic love and the intense sacrifice that comes along with it.
Because the central focus of this series has been around Bella's decision to become a vampire. And each event gives further evidence to why she does love Edward enough to do this. She's grieved in a way that wasn't normal when Edward left her. She weighed the Other Option, Jacob, who was a symbol for an excellent human future and made the decision to be with Edward and change into a vampire anyway. For Bella to have major regrets on this decision, after all that buildup and "proof" of why her love for Edward is enough, would not be in line with the kind of love story it is. It would become another tale about the value of humanity, like Tuck Everlasting, which is not the direction we've been lead for three books.
So, if this is the story I think it is, Bella's age doesn't matter. We as the reader must assume she knows her feelings well enough to know she is making the right choice. SM once commented on the TUGPM thread that people can have different priorities at any age. So waiting for a few years won't make much difference if Bella and Edward's love is as strong as is presented to us. She would make the same choice anyway.
Can Jake really know he loves Bella, when he is only 16?
Going again how the relationships relate to themes again.
The point of Bella/Jake, in my opinion, is to show a more human and natural relationship. He represents another kind of love and the other choice Bella could have had. It would cheapen The Choice that was made if it turned out Jake's love for Bella was shallow, frivolous, and not lasting. Therefore I think it's safe to assume he does love her for that reason alone, again, making age irrelevant.
It does seem naive of Jacob to think he will never love anyone but Bella, and that he will spend his whole life "waiting in the wings". Even Bella, who has the All Consuming type of love for Edward, knows she can be in love with others. And of course he could always imprint and have his perfect soul mate at any time. But I think it is more the desperate situation Jacob is in over Bella, being about to lose her to a vampire, rather than his age/maturity level that makes him refuse to see those other options he has.
do you think this is realistic for real-life teenagers?
Well...it depends on the teenagers involved and the situation they are in. Is it realistic for teenagers to get married at 18 just because they're in love? Well, sometimes and sometimes not. In real life, we don't get themes and plot directions and author preferences that ensure our happy endings. It's best to err on the side of caution. But either way, if people are making decisions about love too young, at least becoming a vampire is not an option.