There are a few ways to approach this which can be used separately or combined.
Option 1. Skip forward
Your question suggests this is not the answer you want without explicitly eliminating it, but it is the fastest and most direct option. You can have 13 years and as much level advancement as you want happen off-screen. You can either work with your characters to compose a brief story of what happens in that time or leave it vague. It gets you exactly where you wish to be, and does so instantly.
Option 2. Make Resurrection Unusually Easy to Get
In DnD, death is not always permanent. Lower level characters may not always have the resources for it, but a patron working behind the scenes could readily arrange for the PCs to find various resurrection options unusually easy to come by. You may want them to notice the strangeness of its availability and they may question why only to learn that it was because of a benefactor later.
Option 3. Ban Character Death Entirely, with or without telling your Players
A. Benefits and Techniques
As the GM, you have control over the entire universe other than the PCs. That is a lot of power to wield to prevent character death, especially in a universe where death is not always permanent.
I have never GMed a 3.5 campaign, but in many other settings (including other editions of DnD), I have outright banned character death up front (with the exception of intentional heroic sacrifices) and it works remarkably well. At least with the groups I deal with, they are much more inclined to get invested in the story of their characters if they know the characters will not die. It is not hard to find in character reasons for why they are kept alive, though the reason does need to depend on the situation.
Remember, sentient opponents often have good reason to prefer a hostage over a corpse so will often refrain from dealing the killing blow if the character can be captured. Many animals that do not plan to eat the thing they are fighting are also content to stop fighting once they achieved their objective (protect their young, drive away the foreign predator, etc.). In other words, there is often good reason the characters will survive a fight even if they lose. That survival may not be without consequences. If they are captured they have to deal with that, but there is good reason.
Also, remember that you are the GM, you can often adjust the difficulty of combat on the fly. If the fight is not going the direction you want, you can often adjust hit points and any un-displayed powers behind the scenes without the players even noticing. If they do notice, it is rarely hard to come up with a good reason for it in story. Perhaps the dragon has old injuries that are hobbling it. Perhaps the lich prepared most of their spells with something other than combat in mind.
As a last resort, there is always Deus Ex Machina. The Cavalry may come riding in. In DnD a deity may literally intervene on a characters behalf. That does not mean that Deus Ex would come without some consequences though. The Cavalry may want a share of the loot. An intervening deity may expect a favor very soon. Done right, it can lead to a new and interesting plot instead of detracting from it. Done very carefully, with the villain escaping instead of being defeated due to the Deus Ex, it can even make the later victory better. Try-fail cycles are a common trope, and Batman in particular routinely faces opponents twice, losing the first time but getting away, then preparing specifically for that opponent and winning the second time. This can seem like you are rescuing them, but if done carefully it will still fit in the plot, and that is why it is the last resort.
In my experience, banning character death works best when you tell the players (but remind them that failure will still have other consequences). It helps them invest in the story. But you can use all of these techniques without informing them.
B. Potential Drawbacks
There are some potential drawbacks to banning character death, particularly when the players know you are doing it. The biggest and most obvious is that it can encourage players to take risks they otherwise wouldn't take. If you are going for a gritty campaign that can be a major problem. However, this is less of an issue if you want your characters to be swashbuckling and going for daring feats.
Even in a gritty campaign, you can mitigate this problem by making sure there are still penalties for losses. Things that might lead to death, but for GM mitigation, might readily lead to loss of equipment instead. Depending on the circumstances this may be thoroughly realistic. If the character is knocked unconscious, one of the enemy might just grab their awesome sword in the process and that particular enemy might get away or the sword might be lost in the scuffle. If the GM gives the players an opportunity to flee, they may drop shields or other equipment in the process (The famous quote from Sparta of "Come back with your shield or on it" originates from the fact that a fleeing army would almost always abandon at least their shields. The dead were also often carried on their own shields). Etc.
Another major problem is that you might find yourself forced to invoke deus ex machina. That can make the game feel less realistic and force you to come up with sometimes extreme events on the fly. The best way to mitigate that is use Deus Ex only as a last resort. You can also make it feel a bit less like Deus Ex if you foreshadow ways a powerful entity might step in on your player's side. If the players know a deity or archmage wants them to succeed it will at least make plot sense for something nearly miracluous to happen that helps them out if needed. And remember that Deus Ex does not need to be free from consequences. A deity may expect a favor or sacrafice in return, a patron may reduce rewards, etc.
