I have several bags of raked leaves that have been under my porch for about 5 years. Would they still make decent soil.

Sure. If they were bagged wet, they might already be all rotted ( in a good way). If they were bagged dry, they might look about the same as the day they were bagged.

What are you thinking of doing with them?

They've gotten wet because of a few holes in the bags over the years. I have a huge tree in my yard with roots running all through the yard, including the area along my porch where I want to plant something, flowers, herbs, etc. I've got some planks to put around the area and thought I would add the leaf/mold inside the boxed-in area for more soil, bringing the plants up and over the root system of the tree. If I can deepen the soil, perhaps the tree roots won't disrupt the plant growth. My thoughts anyway.

It sounds like good material to top dress the root area.
You won't want to add too much actual dirt over tree roots, it is not good to bury them much deeper than what they have grown as. And whatever you do, the tree roots are quick to adapt and compete with any new plants.
Spring bulbs, hostas, and short season spring flowers like columbine or pansies would probably do ok around trees.
I have actually a variety of plants under a couple of maple trees, so plants can grow there, but won't be the sort of bright, colorful all summer, flower bed that you can have in sunnier, more open areas.

The tree roots stretch all around the yard and are not specifically under the tree/drip line but are near the surface of the surrounding ground all over the yard, including sunny areas. That shouldn't create a problem, should it? I'm talking a huge tree with huge, far-spreading roots. The growing area I'm planning is in the shade of the tree in early morning and late afternoon/evening. The area where I want to use the soil is next to my house (mobile home) and in the sun from mid morning and throughout the afternoon into early evening.

I think I have the idea of what you are planning and think the trees will be fine.
When you plant, if you find tree roots, just be aware, they will quickly try to invade the planting holes, and new plants might need more care/water.
I would plant sun-loving plants there.

I should have sent you a photo, but I think you have the idea. I love my tree but would very much like to have flowers around the porch area and have also used aluminum tubs for herbs and such. As a couple of them have proved to be pretty aggressive (thyme and mint), I'll keep them in containers, maybe using some of the leaf rot as soil. Otherwise, the ground around my mobile home and porch is pretty dry and solid with only large tree roots breaking the surface. Thank you for all of your information. I feel a lot more confident in pursuing my vision for the floral area.

p.s...now that you have answered the main questions about the tree rot and possibilities, I have a million more questions! I have a very limited income, but since I own my mobile home, I have years to get my yard into the shape I envision. I don't have any plan as yet, just the flowers and herbs growing in the porch area. I want my home to look pretty, and I will probably plant the herbs in planters along the porch railing (easily accessible when I'm cooking). I'm so excited!!

There are many folks here who will be happy to answer any new questions.