The biggest difficulty for representing this concept within the rules is an obvious one: why is this character 1st level? If this character's been on all these adventures and quests, why isn't she a much higher level? Isn't that a little weird?
Now, you could simply ignore the mechanical side of things and just accept that little wrinkle, but where's the fun in that? After some brainstorming, I've worked out two easy ways to somewhat reconcile the character's backstory with their mechanics. While I'm sure there are other ways to do this, these are my personal favorite and I feel they work the best.
A NEW PATH
Yes, your character has experienced these different adventures and has numerous stories to tell. However, the reason they're a 1st level character is they've decided to follow a new path for some reason, starting over with a different class. For example, let's say you've decided to create a grizzled veteran of numerous battles that happened to find religion and decided to take up the cloth. She sets aside her sword and spends a few years learning these new divine rites. It would make sense for the character to be 1st level because she's starting from the beginning with something else.
This concept takes things down a different road. While this character used to be a great adventurer, something caused her to retire. She's spent years living a regular life, her skills waning due to not using them as often as she did before. However, something drags her back into the life. Although she still has that knowledge and skill buried inside her, she's out of practice and it's going to take her awhile to get back to her previous level of power.
I'll admit, these are not perfect fixes. Mostly, they're ways to help you suspend your disbelief. If you want to take things further, you could rework your ability scores to better represent the character's age. Maybe the warrior has a lower Dexterity score, representing her age's effect on her reflexes and joints. Maybe she has a higher Wisdom as well, representing her different experiences and how they've affected her.
You can make almost any concept work as long as you sit back, take some time, and really think about it. There's always a twist or an alteration you can make, you just have to look for it.