Q2. What's the difference between the following two degrees of far and old;

1. far farther farthest
2. far further furthest


1. old older oldest
2. old elder eldest

Re "far":

[This] involves real and abstract distance; that is, when you are talking about inches, feet and miles—things that can be measured with a ruler—use “far, farther, farthest.” When your distance is abstract—not measurable in a physical way—use “far, further, furthest”:

I drove farther than I did yesterday.
I am further along than Jack when it comes to math. [source]

Re "old":
Elder and eldest are chiefly used for comparison within a family. Also, "elder" cannot be placed before than. [source]