A very precise age determination can be difficult in many cases. There is no simple rule or certain procedure.
Relatively speaking, the safest indications of age are not simply samples and colors, but material, structure and the total condition of the carpet's weave. One compares the motives into the finest details with illustrations of similar pieces, and in case of uncertainty it is advisable to do a material analysis.
An important date of reference in this is the year 1860 when aniline dyes were first used in the Orient. 'Antique carpets' should be produced before this time.
Roughly, one can use the following scale, published by the BVOI (Federal Association of Oriental Carpet Importers):