A drawing or painting on brittle yellowing paper may not be an antique. The process that causes aging in works on paper can damage the art in a matter of weeks or months rather than decades.
It is the acid in the paper that causes it to gradually discolor yellow or brown with age. If the work on paper is framed properly the process will be slower but never completely stopped. The best way to prevent the discoloration is to use acid free paper and to prevent the paper from contacting other acids.
Paper is made of cellulose which is an vegetable material from cotton, wood or flax. When cellulose ages it will discolor and become more brittle but the better quality the paper, the more slowly the process will occur.
Rag paper, made from linen fibers (flax) that has a non acidic sizing is the most durable. Sizing is the gelatin that seals the paper to make it less absorbant. Cotton also has cellulose but the cotton plant fibers are shorter and the resulting paper is not as strong. Papers such as newsprint, cardboard, or inexpensive drawing paper are made from wood pulp are acidic and will yellow quickly
The lignen that holds the wood cells together is very acidic but innexpensive papers are also often sized with acidic sizing. Matboards sometimes have a buffering agent added to counteract the acid bearing lignen in the board. These are sometimes called alpha-cellulose boards and are considered stable.
Even good quality acid free paper will discolor if it is not protected. You have to be sure the paper is framed with acid free materials and keep it away from sunlight and air.
Acidic framing materials include corrugated cardboard, masking tape, wood backing or rubber cement. If any of these touch the acid free paper, the acid will migrate to the paper and deterioration has begun in earnest. Ultraviolet light provides the energy that fuels the deterioration process as long paper fibers are broken into small brittle fragments and begin to turn yellow.
The sulphur dioxide in the atmosphere which is worse in cities or other areas of air polution, can react with the moisture in the air to form sulphuric acid (acid rain). Since paper has a lot of moisture, it absorbs this acid and begins to turn yellow. In most areas moisture that seeps into framed art will also cause mildew or foxing spots. Works of art on paper have to be protected under glass or plexiglass and sealed on the back to keep moisture out.
Use acid free papers for your art. Frame it carefully with buffered or acid free framing materials. Protect the art from excess sunlight or open air. The dangers that cause deterioration of art cannot be entirely prevented, but these measures will ensure that the art will not die before its time.